teinase (Asch and Dresden, 1977, Comp. Biochem. Physiol. 58B: 89-95). Cleavage products from the action of cercarial proteinase, trypsin, or chymotrypsin on the 66 and 60K polypeptides of keratin were identified by SDS polyacrylamide gel electrophoresis (Weber and Osborn, 1969, J. Biol. Chem. 244:44064412). First, 120 ,tg of substrate (purified 66 or 60K keratin) and 0.4 ,ug of purified enzyme were incubated at 22 C for 1, 4, or 20 hr in 90 ,ul of the 100 mM Tris/HCl buffer described above. The reaction mixture was then boiled for 5 min in the presence of 1% mercaptoethanol, 2% SDS and 4 M urea. Then 50 Atg of substrate were subjected to electrophoresis on a 9% polyacrylamide gel (10 x 0.6 cm), and the gel was stained with Coomassie brilliant blue, R250. The purified cercarial proteinase degraded whole purified keratin, and this activity was inhibited by a1-proteinase inhibitor (Fig. 1). The extent of keratin degradation was limited by the amount of enzyme present and stability of the enzyme at 37 C. The proteinase also degraded both the 66 and the 60K keratin polypeptides. Cleavage products were visible after 4 hr of incubation. By 20 hr, cleavage products ranging from 18,000 to 50,000 mol. wt. were visible (Fig. 2). The cleavage pattern differed from that observed with trypsin or chymotrypsin at 1, 2, 6, and 20 hr. Our results, using purified substrate and purified, secreted enzyme, confirmed previous reports suggesting that cercariae of S. mansoni can enzymatically degrade skin keratin. Our analysis of keratin cleavage products by SDS polyacrylamide gel electrophoresis indicated that the sites of cleavage by cercarial enzyme differ from the sites of cleavage by the serine proteinases trypsin and chymotrypsin. Aside from its biologic role of facilitating cercarial penetration of skin, cercarial proteinase may be useful in studying the structural domains of keratin. Supported by The Edna McConnell Clark Foundation, The Burroughs Wellcome Fund, and National Institutes of Health (AM 12433).