A stimulation by zinc ions of the hydrolysis of ATP by prostasomes prepared from human semen has been observed. Stimulation was maximal at a Zn2+/ATP stoichiometry of 0.5/l, and increasing this ratio resulted in a gradual decrease in ATPase activity. The pH optimum was 6.0. The apparent Km for Zn2+-dependent ATPase was 0.43 mmol/l and apparent Vmax 5.60 mumol/mg protein/20 min. Other divalent cations could replace Zn2+ as cofactor more or less effectively in the order Mn2+ greater than Cd2+ greater than Ba2+ greater than Sr2+. Potassium ions produced a further activation of the Zn2+-dependent ATPase system by about 10%. Such a stimulation was also attained to some extent by other monovalent cations as Rb+, NH4+, Li+ and to a lesser extent by Cs+. Orthovanadate in the concentration interval 5-1,000 mumol/l was inhibitory of the Zn2+-dependent ATPase system in a dose-dependent fashion. An aminopeptidase activity was also linked to the prostasomes. This enzyme activity was dramatically inhibited by 2 mmol/l orthophenantroline. A reactivation of the orthophenantroline-inhibited aminopeptidase activity was possible by adding Zn2+ to the reaction mixture. Hence, prostasomes contained ATPase as well as aminopeptidase activities both of which being dependent upon Zn2+. These two activities did not seem to be expressions of an ATP-dependent protease activity associated with prostasomes.