Plasma levels of protein C (enzyme immunoassay), albumin (electrophoresis), and transthyretin (radial immunodiffusion) were measured in 15 patients with the compensated hepatosplenic form of schistosomiasis and in 10 healthy volunteers. Plasma levels of protein C were below normal in 47% of the schistosomiasis patients; this deficiency could be explained by diminished hepatic synthesis since it occurred in conjunction with low plasma levels of albumin and/or transthyretin. In 33% of the schistosomiasis patients, plasma levels of protein C were below 0.5 U/ml, a value which has been associated with thrombotic disease. Protein C deficiency may explain the unexpectedly low incidence of hemorrhagic episodes, as well as the occurrence of portal vein thrombosis that is not infrequent in these patients.