Protease-containing preacetabular gland secretion can be collected from cercariae of Schistosoma mansoni by stimulation with penetration-inducing skin surface lipid, but the method lacks quantitative control because of variability of the lipid. Two commercially available free fatty acid fractions of skin lipid active in stimulating cercariae to penetrate skin, linolenic and linoleic acids, were substituted for skin surface lipid in a technique which provides an improved method of secretion collection. The pattern of protease activity of secreted enzyme(s) was followed throughout patency of infection of a group of snails with the same date of exposure. Day-to-day variability was a characteristic feature of all parameters studied. Major trends were elevated enzyme activity and cercarial emergence during midpatency.