Certain types of somatic cells, as well as follicular cumulus cells associating with mammalian oocytes, are known to produce beneficial effects on in vitro fertilization and preimplantation development of mammalian eggs when they are present in oocyte culture medium. To investigate the nature of the effects of somatic cells, the resistance of mouse oocytes against chymotrypsin treatment was examined after culture within various cell conditioned media. When mouse oocytes matured for 17-18 hr in the presence of cumulus cells were treated with 1% chymotrypsin, half of them remained still alive even after 240 min (t50>240.0). In contrast half of mouse oocytes cultured without cumulus cells underwent degeneration within 65.0 min (t50= 65.0+/-13.2 min) of the same treatment. To see if the effects were due to the secretory products of cumulus cells, mouse cumulus cells were cultured for 20 hr in medium containing 0.4% BSA and the supernatant of culture medium (conditioned medium) was taken. After maturation in the cumulus cell conditioned medium, mouse oocytes exhibited t50 = 190.0 +/-10.8 min upon chymotrypsin treatment whereas half of oocytes cultured without conditioned medium degenerated within 25.5 min. Human granulosa cell conditioned medium gave similar effects such that oocytes matured in conditioned medium exhibited t50 = 183.3+/-19.1 min while t50 of control group oocytes was 60.0+/-6.8 min. Oocytes matured in vero cell conditioned medium exhibited t50 = 196.7+/-8.8 min. On the other hand, amniotic cell conditioned medium resulted in the chymotrypsin resistance of t50 = 80.0+/-8.4 min which was not statistically different from the control value of t50 = 48.0+/-13.2 min. Based upon these results, it is suggested that certain somatic cell types including cumulus cells might change the biochemical properties of mouse oocyte membrane during meiotic maturation as revealed by the enhanced resistance against chymotrypsin treatment. Such effects of somatic cells appear to be mediated via the secretory products rather than direct communication between somatic cells and oocytes.