Maternal fertility decreases with aging, possibly owing to qualitative changes in the egg itself and the egg-housing condition of the female reproductive tract. Since sperm first interact with the egg-coating structures, age-dependent alterations of egg-coating structures may be related to reduced fertility in aged female mammals. Quantitative genetic and biochemical studies of specimens prepared from aged mammals have revealed altered expression patterns of antioxidant- and apoptosis-related proteins in cumulus cells, and a substantial decrease in the amount of ZP-constituent glycoproteins. Histochemical studies have demonstrated an increase in the number of apoptotic cumulus cells and significant alteration in the appearance of ZP with irregular plaques in aged specimens. Biophysical studies have shown that both susceptibility to digestion with protease and mechanical stiffness are reduced in the ZP aged in vitro. We recently characterized a novel suppressive factor of fertilization, dicalcin, in the cumulus-oocyte complex. The expression level of dicalcin in the cultured normal human fibroblasts increases with the passage of time, which implies an age-dependent increase in its expression in the normal female reproductive tract. The potential increase in dicalcin expression with aging would represent a qualitative change of the egg-coating structure, augment its inhibitory role on fertilization, thereby causing decreased fertility in aged female mammals.