Caesalpinia sappan extract from a study of screened Chinese herbal medicines was found to be a potent agent for the inactivation of human sperm in vitro. Exposure of sperm from healthy donors to this agent showed remarkably reduced sperm motility. The antimotility effect of Caesalpinia sappan is concentration-dependent and about 2.5 mg/ml is required to reduce motility to 50% the control medium (EC50). This result suggests that this traditional Chinese herbal medicine possesses an antimotility effect on human sperm in vitro and has the potential of becoming in the future a new and acceptable male oral contraceptive.