Sodium dodecyl sulfate (SDS)-lysates of E. histolytica trophozoites were analyzed by electrophoresis in simple and gelatin-containing ("substrate") SDS-polyacrylamide gels. In simple gels, boiled lysates with para hydroxymercuribenzoate (pHMB) had a complex pattern of apparently undegraded proteins; boiled lysates without pHMB showed a major 30 kDa and four minor (43, 46, 63 and 117 kDa) proteins, whereas unheated lysates displayed only the 117 kDa protein. Using substrate gels no gelatinases were detected in heated lysates; unheated lysates without pHMB showed a major 30 kDa and three minor (33, 46 and 68 kDa) gelatinases, whereas those with pHMB presented a major 56 kDa and two minor (70 and 105 kDa) gelatinases. Three caseinase peaks were separated by Sephadex G-75 chromatography from unheated lysates: peak I contained 46, 56 and 117 kDa pHMB-sensitive gelatinases and peaks II and III contained smaller pHMB-resistant caseinases. We conclude that proteins remaining in lysates after SDS-induced proteolysis appear to be mainly proteases relatively resistant to self-digestion whose type and amount changes with the conditions of lysis and the presence of inhibitors; this is exemplified by the finding of the major gelatinase of lysates with pHMB being larger (56 kDa) than in lysates lacking the inhibitor (30 kDa).