Hemoglobin in finished fetal bovine serum results from the lyses of red blood cells during the transportation and processing of blood into serum. When collection techniques were not as good, hemoglobin concentration was used as an indicator of the quality.
There are no reported effects on cell cultures from hemoglobin concentrations of less than 100 mg/dL. Normal levels of total hemoglobin in finished fetal bovine serum range from 10–30mg/dL. If you are using a Certificate of Analysis to compare the quality of fetal bovine serum and are considering hemoglobin as a factor, call the manufacturer; ask about the assay they are using and their opinion regarding hemoglobin's effect on cell cultures.