The present study was conducted to examine the penetrability in vitro of immature porcine oocytes with or without cumulus cells. Porcine oocytes were cultured for 0-36 h, at 39 degrees C in 5% CO2 in air, in modified tissue culture medium 199 (TCM-199B at pH 7.4) supplemented with 10 IU eCG/ml, 10 IU hCG/ml, and 1 microgram estradiol-17 beta/ml. At various times after the beginning of culture, some oocytes were freed from the cumulus (cumulus and corona cells), and cumulus-intact or cumulus-free oocytes were inseminated with cryopreserved ejaculated spermatozoa in TCM-199B (pH 7.8) containing 5 mM caffeine. When cumulus-free oocytes were examined 14 h after insemination, high proportions (69-84%) were penetrated and there were no significant differences among different periods of maturation culture. The incidence (47-68%) of polyspermy and the number (1.7-3.1) of spermatozoa that penetrated per oocyte were also not significantly different among oocytes cultured for 0-36 h. In cumulus-intact oocytes, however, the first evidence of penetration (15%) was observed in oocytes cultured for 6 h. The penetration rates increased significantly as the period of culture was prolonged up to 24 h. Almost all (95-100%) oocytes were penetrated when they were inseminated 24-36 h after the beginning of culture, by which time the cumulus masses showed moderate to complete expansion except for the corona radiata. A similar correlation was also observed for incidence of polyspermy and number of spermatozoa penetrated per oocyte. The presence of well-expanded cumulus around the oocyte during fertilization promoted male pronuclear formation in penetrant oocytes.(ABSTRACT TRUNCATED AT 250 WORDS)