We believe we will make these investments and remain well-positioned to achieve our operating margin target of 22.5% in 2024.
Quarterly revenue surpassed $900 million for the first time and a $914.6 million in the second quarter of 2021, represented a 34% increase over last year.
Organic revenue growth of 24.1% was increased by approximately 8%, when compared to last year's COVID-19 impact in the second quarter of 2020, with the greatest impact in the Research Models and Services segment.
The operating margin was 20.8%, an increase of 350 basis points year-over-year.
Notwithstanding this favorable year-over-year comparison, we were pleased with the margin progression in the first half of the year and are on track to achieve a full year operating margin of approximately 21% or 100 basis points higher than last year.
Earnings per share were $2.61 in the second quarter, an increase of 65.2% from $1.58 in the second quarter of last year.
This result widely exceeded our prior outlook of more than 50% earnings growth for the quarter, primarily as a result of the exceptional demand environment.
Based on the second quarter performance and our expectation for sustained demand through the remainder of the year, we are increasing our revenue growth and non-GAAP earnings per share guidance for 2021.
We now expect organic revenue growth in a range of 13% to 15%, 100 basis point increase from our prior range.
Non-GAAP earnings per share are expected to be in the range from $10.10 to $10.35, which represents 24% to 27% year-over-year growth and an increase of $0.35 at midpoint from our prior outlook.
Revenue was $540.1 million in the second quarter, an 18.1% increase on an organic basis over the second quarter of 2020, driven by broad-based demand for both Discovery and Safety Assessment Services.
The DSA operating margin increased by 30 basis points to 23.5% in the second quarter.
Foreign exchange reduced the DSA operating margin by 150 basis points in the quarter as revenue and costs are not naturally hedged at certain DSA sites, including our Safety Assessment operations in Canada.
We continue to expect the DSA margin will be in the mid-20% range for the year.
RMS revenue was $176.7 million, an increase of 44.5% on an organic basis over the second quarter of 2020.
Approximately 33.4% of this growth was attributable to the comparison to last year's COVID-related revenue impact from client site closures and disruptions, which reduced research model order activity.
Adjusted for the COVID impact, the RMS growth rate was above 10% as strong research activity across biopharmaceutical academic and government clients led most RMS businesses to grow above their targeted growth rates.
The RMS operating margin increased to 27.4% from 9.1% in the second quarter of last year.
Revenue for the Manufacturing segment was $197.8 million, a 26.6% increase on an organic basis over the second quarter of last year.
Consistent with the first quarter, Microbial Solutions growth rate in the second quarter was well above the 10% level, reflecting strong demand for our Endosafe Endotoxin testing systems, cartridges, and core reagents in all geographic regions, as well as Accugenix microbial identification services.
The Biologics Testing business reported another exceptional quarter of strong revenue growth that was well above the 20% growth target for this business.
There has been a rapid increase in the number of cell and gene therapy programs in development to approximately 3,000 programs now in the pipeline, with approximately two-thirds in the preclinical phase, which is expected to continue to fuel the strong growth.
COVID-19 vaccine work was also a meaningful driver of Biologics second quarter growth, but the underlying Biologics growth trends remained above the 20% level, even without the incremental COVID-19 testing revenue.
The Manufacturing segment second quarter operating margin declined by 420 basis points to 33.2%.
Coupled with the addition of Vigene in the third quarter, we expect a full year Manufacturing margin slightly below the mid-30% range.
It will also allow us to achieve our longer-term financial targets of low double-digit organic revenue growth and an average of approximately 50 basis points of operating margin improvement beyond 2021.
Organic revenue growth of 24.1%, including 8% related to last year's COVID-19 impact and operating margin expansion of 350 basis points, were the primary drivers behind earnings-per-share growth share growth of 65.2% to $2.61.
Based on our strong second quarter results and expectations for the underlying strength of demand to continue, we have increased our full year financial guidance and now expect to deliver organic revenue growth in a range of 13% to 15% for the full year.
Primarily as a result of the enhanced growth prospects this year, and to a lesser extent, a favorable tax rate, we raised our earnings per share guidance by $0.35 to a range of $10.10 to $10.35, which represents year-over-year growth of 24% to 27%.
Including the acquisitions of Cognate and, more recently, Vigene Biosciences, Manufacturing's reported revenue growth rate is expected to be in the low to mid-40% range.
With regard to operating margin, our expectations for segment contributions remain mostly unchanged from our prior outlook, with the RMS operating margin meaningfully above 25% for the full year, DSA in the mid-20% range and Manufacturing slightly below the prior mid-30% outlook, principally reflecting the addition of Vigene in late June.
Lower unallocated corporate costs contributed to the second quarter margin expansion, totaling 5.6% of revenue or $51.2 million in the second quarter, compared to 6.1% of revenue last year.
We continue to expect unallocated corporate costs to be in the mid-5% range as a percentage of revenue for the full year.
The second quarter non-GAAP tax rate was 20.4%, representing a 60 basis point decline from 21% in the second quarter of last year.
or the full year, we are reducing our tax rate outlook to a range of 19.5% to 20.5% from our prior outlook of a tax rate in the low 20% range, principally driven by a higher benefit from stock-based compensation.
Total adjusted net interest expense for the second quarter was $20.8 million, an increase of $3.7 million sequentially and $1.7 million year-over-year, due to higher debt balances primarily to fund the Cognate acquisition.
At the end of the second quarter, we had an outstanding debt balance of $2.7 billion, representing gross and net leverage ratios of about 2.5 times.
For the full year, we now expect total adjusted net interest expense to be slightly below our prior outlook in a range of $82 million to $85 million, primarily reflecting the accelerated debt repayment.
Free cash flow was $140.2 million in the second quarter, an increase of 3.5% over the $135.5 million for the same period last year.
In view of our robust results in the first half of the year, we have increased our free cash flow outlook by $65 million and now expect free cash flow of approximately $500 million for the full year.
capex was $46.4 million in the second quarter last year, compared to $26.8 million last year.
We continue to expect capex to be approximately $220 million for the full year.
Accordingly, we expect organic revenue growth in the low to mid-teens range and reported revenue growth in the low 20% range.
We expect low double-digit earnings-per-share growth when compared to last year's third quarter level of $2.33.
I will remind you that the DSA operating margin in the third quarter of last year included a 50 basis point benefit from a discovery milestone payment, which will impact the year-over-year comparison.

Summary:
Quarterly revenue surpassed $900 million for the first time and a $914.6 million in the second quarter of 2021, represented a 34% increase over last year.
Earnings per share were $2.61 in the second quarter, an increase of 65.2% from $1.58 in the second quarter of last year.
Based on the second quarter performance and our expectation for sustained demand through the remainder of the year, we are increasing our revenue growth and non-GAAP earnings per share guidance for 2021.
We now expect organic revenue growth in a range of 13% to 15%, 100 basis point increase from our prior range.
Non-GAAP earnings per share are expected to be in the range from $10.10 to $10.35, which represents 24% to 27% year-over-year growth and an increase of $0.35 at midpoint from our prior outlook.
Organic revenue growth of 24.1%, including 8% related to last year's COVID-19 impact and operating margin expansion of 350 basis points, were the primary drivers behind earnings-per-share growth share growth of 65.2% to $2.61.
Based on our strong second quarter results and expectations for the underlying strength of demand to continue, we have increased our full year financial guidance and now expect to deliver organic revenue growth in a range of 13% to 15% for the full year.
Primarily as a result of the enhanced growth prospects this year, and to a lesser extent, a favorable tax rate, we raised our earnings per share guidance by $0.35 to a range of $10.10 to $10.35, which represents year-over-year growth of 24% to 27%.