Murine schistosomiasis mansoni is a more rapidly fatal disease in hosts deprived of their cell-mediated immune capabilities. Analysis of the histopathology of the disease under these circumstances indicates that rather than the hepatic granulomata characteristic of the normal infection, the host develops zones of liquefactive necrosis in the liver and intestinal mucosa. These lesions are associated with severe parenchymal cell destruction. Such hepatic and mucosal damage, with subsequent toxemia and septicemia, is presumed central to the altered course of the disease.