A method of collecting samples from the Schistosoma mansoni digestive tract was used to study proteinase activity. Activity against hemoglobin and a low molecular weight synthetic substrate, carbobenzoxy-arginyl-arginyl-7-amino-4-trifluoromethylcoumarin, was demonstrated in the soluble fraction of material regurgitated by S. mansoni adults and was dependent on the addition of a thiol compound, cysteine, to the assays. Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel filtration chromatography (AcA54), two proteins with estimated mol wt of 32,500 and 28,500 were found in the regurgitant and were associated with proteinase activity against both hemoglobin and the synthetic substrate. Homogenates of intact worms showed greater specific activity (synthetic substrate) in the females. Further, in bisected worms proteinase activity paralleled protein content, suggesting that, once secreted into the lumen, proteinase activity was distributed throughout the worm digestive tract.