This study was aimed to evaluate the effect on spermatogenesis of a 62 kDa protein (Rp) isolated from 50% ethanolic extract of the root of Ricinus communis in mice. A dose response study in mice revealed that 25 mg/kg body weight/day was the most effective dose. Swiss strain mature male mice of 30 days old were divided into two group namely control and Rp treated (25 mg/kg body weight/day). The study showed that sperm motility and count were decreased significantly in the treated group as compared to the control. The fertility index of the treated groups was reduced by 100%. The activity of HMG Co A reductase and cholesterol were increased significantly in the treated group. The testicular activities of 3 beta HSD, 17 beta HSD, glucose 6-phosphate dehydrogenase and malic enzyme and the level of serum testosterone were decreased significantly in the treated group. The expression of 3 beta HSD and 17 beta HSD were decreased and the expression of StAR increased significantly in the treated group as compared to the control. Proteolytic digestion of the native protein with trypsin and chymotrypsin showed that the proteolytic cleavage did not affect the spermicidal action of Rp. Hence this study can be concluded that Rp impaired spermatogenesis in vivo by suppressing the production of testosterone. (C) 2011 Elsevier B.V. All rights reserved.