The effect of RU 486 at different concentrations (1, 5, 10, and 20 micrograms/ml) was studied on sperm-egg interaction in vitro in B6D2F1 mice. The in vitro fertilization rate of mouse ova decreased from 77.0% (control) to 50.0%, 28.7%, and 7.5% in the presence of RU 486 concentrations at 5, 10, and 20 micrograms/ml medium, respectively (p less than 0.001). A concentration of 1 microgram/ml did not affect the fertilization rate. A progesterone concentration at equal to or double the concentration RU 486 did not reverse the inhibitory effect of RU 486 on in vitro fertilization, which suggests a progesterone-independent mechanism. Exposure of spermatozoa (for 90 minutes) or ova (for 60 minutes) to RU 486 (20 micrograms/ml) followed by washing with RU 486-free medium before coincubation did not affect the fertilization rate. The presence or absence of cumulus cells did not change the inhibitory effect of RU 486 (20 micrograms/ml) on in vitro fertilization. RU 486 at 5 to 20 micrograms/ml medium was associated with perivitelline polyspermy in nonfertilized ova and enhanced perivitelline polyspermy in fertilized ova.