The roles of proteases in reproduction are discussed. Semen coagulation is controlled by a protease similar to fibrinogen, the ''clotting enzyme,'' a part of the prostate secretion. The proteolytic enzyme that controls semen fluidity is still unknown. Breakdown of clot fragments is accomplished by seminin. Sperm motility is a function of the Kallikrein-kinin system, but it is not yet known which protease is responsible for kinin liberation. Seminal proteases, especially seminin and acrosin, are of particular importance for the penetration of cervical mucus and the facilitation of sperm migration. Acrosin is also of importance in the penetration of the zona pellucida, and is regulated by specific protease inhibitors. A number of other proteases are known to be present in the male genital tract, but their functions are not precisely know. Specific inhibition of 1 of the proteases may be the basis of antienzymatic contraception.