The 'Transwell system' was used to test the response of human spermatozoa to human follicular fluid, progesterone, estradiol and mifepristone. Motility parameters were assessed with computer-assisted sperm analysis. Follicular fluid and progesterone induced significant accumulation of spermatozoa. Changes compatible with an increased progressive and hyperactivation-like motility were obtained with follicular fluid but not with progesterone. Mifepristone eliminated the progesterone-induced accumulation of spermatozoa but had no significant effect on the accumulation of spermatozoa in wells containing human follicular fluid. Furthermore, mifepristone abolished the motility changes effected by follicular fluid. Estradiol had no effect on accumulation or motility of spermatozoa. Human follicular fluid exerted a strong effect on sperm chemoattraction and motility in vitro, while progesterone influenced sperm chemoattraction only.