Bovine oocytes were isolated from ovaries recovered in the slaughterhouse and were culture in vitro for maturation. After maturation, oocytes were divided into 2 groups. one group denuded from the cumulus cells by treating with hyaloronidase and the other group left intact. Both groups were then divided into different sub-groups which were transferred into the fertilization media supplemented with different concentration of fetal calf serum (FCS) and with or without 5mM caffeine or 10 ig heparin/ml and were inseminated by frozen thawed semen. When caffeine was added to the medium. the penetration rates of cumulus-intact oocytes were significantly higher than cumulus- free ones in the presence of 5% and 10% ECS. The proportion of both pronucleus was not significantly different among different concentrations of FCS in cumulus-intact and cumulus-free oocytes. However, the proportion of polyspermy increased b increasing the concentration of FCS (27%. 34%, 58% and 76% for 0, 5%. 10% and 20% FCS respectively). In cumulus-free oocytes. penetration rates were drastically decreased when 20% serum was added to the medium. In the presence of heparin. penetration of cumulus-free oocytes were 1 00% and independent of serum concentration in the doses examined. Polyspermic penetration was higher in cumulus-free oocytes compared with cumulus-enclosed ones. When both chemical (caffeine and heparin) were eleminated from fertilization medium, penetration rates decreased in both cumulus-enclosed and cumulus-free oocytes. It is concluded that: l ) Cumulus cells can induce sperm penetration into bovine oocytes in the prsence of caffeune.2) Heparin has more induction of penetration effect on bovine cumulus — free oocytes in the presence of different concentrations of FCS. 3) Fetal calf serum by itself has not any induction effect on bovine oocytes penetration.