Summary:Thymosin fraction 5 (TF5) enhances interleukin‐2 (IL‐2) production by human peripheral blood mononuclear leukocytes (PBL) when cocultured in the presence of phytohemagglutinin (PHA). This important biological activity of TF5 is not due to the presence of two previously well‐characterized component peptides, thymosin &agr;1and thymosin &bgr;4, but rather to a new, still to be identified component of TF5. Comparison of TF5 with other well‐defined thymic preparations and peptides indicates that this biological activity is unique to TF5 and a closely related extract prepared from porcine thymus tissue. Using a standard human PBL cell population obtained by leukopheresis and cryopreservation, it should now be possible to establish a reproducible bioassay for the isolation and characterization of one or more components of TF5 with the property of enhancing PHA‐induced IL‐2 production by human PBL.