A process for preparing thymosin fraction 5 is disclosed in U.S. patent application Ser.  No.  576,509, filed May 12, 1975, now U.S. Pat.  No.  4,010,148.  In spite of the fact that such process utilizes an ultrafiltration step, because of subsequent steps, the resulting product is not completely endotoxin and pyrogen free. 
The use of ultrafiltration to effect removal of pyrogens from injectable drug preparations has been reported.  See for example Chemical Abstracts, Vol.  79, Issue 22, Section 61, Abstract No.  129041; Chemical Abstracts, Vol.  80, Issue 20, Section 63, Abstract No.  112675 (based on Japanese Pat.  No.  73 35445); and Chemical Abstracts, Vol.  83, Issue 20, Section 63, Abstract No.  168487 (based on Japanese Pat.  No.  75100220). 
Additionally, Rubio and Lopez, Appl.  Microbiol.  23, 211 (1972) have disclosed the purification of endotoxins from Pseudomonas aeruginosa using a membrane with molecular weight cut off of 100,000.  The concentration of endotoxin in the permeate was, however, not reported.