In modified Tyrode's solution, 17 beta-estradiol at concentrations between 0.1 microgram/ml and 320 nmoles/ml was effective in increasing human spermatozoal forward migration. 17 alpha-Estradiol, although structurally similar to 17 beta-estradiol, had no effect on human spermatozoal motility. DL-Norgestrel at concentrations between 0.1 migrogram/ml and 320 nmoles/ml inhibited spermatozoal motility. These stimulatory and inhibitory effects were not observed when fasting human blood serum was used as a penetration medium in place of the modified Tyrode's solution. Also, the motility of spermatozoa suspended in fasting human blood serum was better than that of spermatozoa suspended in modified Tyrode's solution or in seminal plasma. These observations indicated that there is a component(s) of fasting human blood serum which increases spermatozoal motility and can counteract the activation or inhibition of spermatozoal motility by 17 beta-estradiol or DL-norgestrel at the concentrations used here. It has been reported that steroids can affect migration of human spermatozoa in vivo and in vitro, as well as spermatozoal metabolism and motility. In the present study, samples of human semen from normal healthy donors were liquefied at 37 degrees C and used within 30 minutes after collection. Only those samples with motility and normal morphology greater than 80% were used. The samples were washed twice in modified Tyrode's solution containing either steroids (17 alpha and beta estradiol, and DL-norgestrel) or no steroids (controls). Fasting human blood sera were collected and used for penetration test. The results show that human spermatozoa can be activated by 17b-estradiol at a wide range of concentration and that 17 a-estradiol does not affect migration. The synthetic progestin DL-norgestrel at concentrations between 0.1 mcg/ml and 320 nmoles/ml was found to inhibit motility. Steroidal effects on spermatozoan migration in vitro occurred after removal of seminal plasma, suggesting that steroids exert their effects directly on the spermatozoa rather than through the seminal plasma. The stimulatory and inhibitory effects were not observed when fasting human blood serum was used as a penetration medium. The motility of spermatozoa suspended in fasting human blood serum was also found to be better than when modified Tyrode's solution or seminal plasma was used as the medium. The findings suggest that certain factors in the fasting blood serum enhance spermatozoal motility, but has no effect on maintenance of spermatozoal viability.