Total and free testosterone (T) and estradiol (E2) were determined by a radioimmunoassay technique in 30 normozoospermic and oligozoospermic human semen samples. The mean amounts of total T ranged from 25.1 +/- 7.4 to 32.5 +/- 12.2 ng/100 mL, and those of the free hormone from 7.0 +/- 2.8 to 7.9 +/- 2.3 ng/100 mL. The values for E2 were 10.5 +/- 4.4 to 11.2 +/- 4.5 pg/mL and 2.3 +/- 1.1 to 3.2 +/- 2.6 pg/mL, respectively. There was no correlation between sperm density of semen and the levels of T and E2. It is suggested that the nonspecific binding of T and E2 in semen provides a physiological reservoir for active, free T and E2 and that an appropriate ratio of free T to free E2 in semen may be crucial in the processes that lead to fertilization.