The anti-implantation and antiovulation effects of castor bean extract (CBE) and ricin A-chain (RAC) were evaluated in rabbits. Both CBE and RAC, administered intraperitoneally on days 5-9 of pregnancy, exhibited a pronounced decrease in maternal body weight gain and in death of all fetuses. A significant (p < 0.01) decrease of implantation sites resulted after rabbits were treated with RAC on the first 6 consecutive days of pregnancy. When female rabbits were treated with RAC for 10 consecutive days followed by human chorionic gonadotropin (hCG) (50 IU/kg intravenously), there was a 30% reduction in the number of corpora lutae. These data clearly indicate that CBE and RAC possess potent effects on implantation and ovulation in rabbits. The protein contents of castor bean extract, separated by polyacrylamide gel electrophoresis, revealed the presence of several protein bands, ricin toxin being a major constituent of the extract.