Abstract The effects of various salts on the proteolytic activity of extracts from Schistosoma mansoni cercariae were tested. Using an Azocoll substrate, stimulation (2 to 2.5-fold) of activity by the monovalent cations Na + and K + was demonstrated, with maximum stimulation at 20–40 m M concentrations. The divalent cations Mg 2+ and Ca 2+ stimulated proteolytic activity at low concentrations (between 0 and 10 m M) but inhibited activity at higher concentrations. The divalent cations Zn 2+, Cu 2+, Fe 2+, and Co 2+ were inhibitory even at very low concentrations. The results presented here are discussed in relation to previously described ion effects on cercarial infectivity.