Enzyme linked immunosorbent assays were used to measure serum levels of immunoreactive thymosin alpha 1 and thymosin beta 4 in 681 healthy adults (range 18 yr-101 yr). Immunoreactive thymosin alpha 1 was found to occur at a geometric mean of 540 pg/ml with a range from 252 to 1158 pg/ml. There were no differences in the levels when analyzed according to race, sex or age. Immunoreactive thymosin beta 4 was measured at a geometric mean of 12.6 ng/ml with a range from 6.9 to 23.0 ng/ml. There were no differences in the levels when analyzed according to race or sex, however, there was a slight upward trend with increasing age. This data may be useful as a reference when monitoring clinical studies, determining the kinetics of drug metabolism and distinguishing patient groups with elevated levels of either of these peptides.