Fertilization and cleavage rates of human cumulus-intact oocytes incubated in vitro for 36-48 h with normal spermatozoa tended to be higher than those which were cumulus-denuded (73 versus 68%; 68 versus 56%, respectively); however, the difference was not significant. Nor were these differences significant when using sperm samples of various qualities (normozoospermic samples: 75 versus 70% fertilized oocytes; asthenozoospermic: 66 versus 64%; oligozoospermic: 64 versus 56%; oligoasthenozoospermic: 35 versus 33%). The beneficial effect of the human cumulus oophorus on the binding of human spermatozoa to denuded hamster oocytes and on head decondensation of human spermatozoa observed after 2 h of incubation (9.3 versus 7.0 bound spermatozoa per oocyte, P less than 0.05; 0.5 versus 0.3 decondensed sperm heads per oocyte, P less than 0.02) disappeared after 6 h. A protective effect of the cumulus oophorus on hamster oocytes preincubated in medium containing 50% human preovulatory follicular fluid was observed in the sperm penetration assay (fertilization rate of cumulus-intact: cumulus-denuded oocytes, 26 versus 13%, P less than 0.05) and confirmed using fluorescein diacetate stain (cumulus-intact oocytes: 86 versus 100% vitality, non-significant; cumulus-denuded oocytes: 64 versus 100%, P less than 0.01). These data suggest the accelerating effect of the human cumulus oophorus on fertilization in its early stages. Furthermore, the cumulus plays an important part in protecting the oocyte against adverse environmental influences.