The protein "SF-50", isolated from crude extract of spermatheca gland of a marine Gastropoda mollusc, Telescopium telescopium, has been found to prevent entry of human spermatozoa at the dose of 6 and 5 mg/ml. No implantation site was observed in rats when 10 mg of "SF-50" was administered in the uterus and the sites were significantly less (P< 0.001) in the group treated with 5 mg than those in BSA control. Continuous diestrous phase up to 23 to 50 days were observed in the actively immunized female rats with "SF-50". There was no degenerative change or any significant variation in the number of corpus luteum, implantation sites or litter size in actively immunized rats as compared to the controls. The anti "SF-50" rat serum showed homogeneity and identity with the corresponding antigen. No toxic effect of "SF-50" was observed in ocular irritation test in rabbits. Present findings indicated a plausible topical vaginal contraceptive from marine bio-active substances.