Potent in vitro spermicidal activity of parabens against human spermatozoa was demonstrated in this study. The "pass" point concentration of the four parabens--methylparaben, ethylparaben, propylparaben, and butylparaben, at which all spermatozoa were immobilized and no immobilized spermatozoon revived after 30 min incubation in phosphate buffered glucose solution, was 6, 8, 3, and 1 mg/ml, respectively, as tested by Harris' method. These parabens are used as food and pharmaceutic preservatives; less toxicity and side effects were expected for the development of parabens as vaginal contraceptive agents.