High levels of thymosin alpha 1 (T alpha 1) were detected in human seminal plasma and follicular fluid. In the seminal plasma of 19 males studies, T alpha 1 levels varied from 614 to 2,604 pg/mL (mean +/- SD, 1,682.4 +/- 453.9 pg/mL). There was a correlation between the T alpha 1 levels and the total number of sperm in the ejaculate (r = .18) and seminal volume (r = .26). The infertile males, who had low levels of T alpha 1 also demonstrated fewer sperm, reduced motility, and lower semen volume. In follicular fluid collected from 24 follicles of 10 infertile females, T alpha 1 levels varied from 1,019 to 6,384 pg/mL (mean +/- SD, 3,572.8 +/- 1,599.7 pg/mL), which were higher when compared with the corresponding serum levels (mean +/- SD, 1,666.9 +/- 1,378.9 pg/mL). T alpha 1 levels present in follicular fluids which had "immature" oocytes were lower when compared with follicular fluids which had "intermediate" or "mature" oocytes. The immunoreactive T alpha 1 present in seminal plasma of males and in the follicular fluids of females may be involved in some aspect of germ cell maturation and function. The measurement of T alpha 1 levels may be useful in the diagnosis and treatment of male and female infertility, and also as a novel marker for the assessment of maturity of oocytes required for in vitro fertilization-embryo transfer.