The proportion of eggs undergoing fertilization in vitro was compared between intact (Fig. 1, A) and denuded (Fig. 1, B) eggs using capacitated (pre-incubated) spermatozoa.<BR>Superovulated eggs were obtained 1617 hr after injection of HCG from adult female mice of ICR-JCL strain. They were liberated from the oviduct under warm mineral oil in a plastic petri bish containing 0.4 ml of incubation medium (modified Krebs-Ringer-bicarbonate solution). Immediately, one group of eggs with cumulus cells was treated with hyaluronidase by adding 0.08 ml of a hyaluronidase solution (900 unit/ml) to the incubation medium containing egg clots. After incubation for about 7 min, the denuded eggs were transfered through three changes of the medium to remove the cumulus cells. Another group of eggs was not treated with hyaluronidase and used for fertilization immediately after recovery.<BR>The sperms used were recovered from cauda epididymis of adult males and suspended in the incu-bation medium in a concentration of 3, 0006, 000/mm³. The sperms were then incubated at 37°C under 5% CO₂ in air for 1 hr before they were used for insemination.<BR>At the time of insemination, a drop of pre-incubated sperm suspension (ca. 6.310.3 mm³) was added to the medium containing intact or denuded eggs. Final concentration of sperm was estimated at 50400/mm³.<BR>After insemination, the eggs and sperms were incubated at 37°C in a humid atmospher of 5% CO₂ in air for 4 to 5 hr. At the end of incubation, the eggs were placed on a glass slide, compressed gently under a cover slip and examined for sperm penetration through the zona pellucida, sperm entry into the vitellus and the formation of male and female pronuclei.<BR>The results indicate that there is no significant difference in the proportion of penetrated eggs (Table 3), the proportion of eggs undergoing fertilization and the rate of polyspermy (Table 2), whether the eggs are denuded or not. The proportion of eggs undergoing fertilization was 71.3% in denuded eggs as compared with 84.7% in intact ones. The numbers of spermatozoa found in the penetrated eggs also showed similar distribution between two groups.<BR>It is considered from these results that the presence of cumulus cells does not play a role for the sperm penetration of mouse eggs in vitro.