These experiments were carried out to investigate whether zona hardening affect the efficiency of in vitro fertilization in mouse oocytes. The soluble properties for zona pellucida of oocytes matured in vivo, aged oocytes, and ovarian oocytes matured in vitro have been analyzed with proteolytic enzyme, 3mg/ml of $alpha$-chymotrypsin. The mean solubility(t50) for the zona of unfertilized oocytes, oocytes not fertilized at the first inseminati and in vitro produced zygotes were 10.1, 20.3 and 32.3min., respectively. The t50 for zona lysis of fertilized oocytes was significantly difference than those observed for unfertilized oocytes and oocytes not fertilized at the first insemination(P<0.01). In addition, the t50 of zona in ovulated oocytes with and without cumulus cells incubated for 0, 3, 6, 9 and 12 hr in vitro, t50 were 13.9, 11.1, 20.7 and 28.0min., and 22.3, 21.0, 30.0 and 33.5min., respectively. In these experiments, the zona pellucida showed a gradual increase in resistance to dissolution by $alpha$-chyjotrypsin with in vitro aging for more than 6 hrs. This effect was greater in cumulus-free as compared to cumulus-intact oocytes. Finally, in cumulus-intact and cumulus-free ovarian oocytes matured for 0, 5, 10 and 15 hr in vitro the t50 of zona pellucida were 3.0, 10.6, 18.4 and 24.5 min., and 3.0, 14.0, 26.2 and 32.0 min., respectively. Clear differences in solubility between the zona pellucida of oocytes matured in vivo and in vitro. This data were found suggest that under in vitro conditions there is a gradual change in the soluble properties of the zona pellucida, particularly in the absence of the cumulus cells.