Protein antigens present in the hepatic lesions of mice infected with Schistosoma mansoni were identified by use of fluorochrome-conjugated monoclonal antibodies. All 26 monoclonal antibodies used in these experiments recognized antigenic determinants expressed by both cercariae and adult worms; 23 of these epitopes were also expressed by S. mansoni eggs. These results suggest considerable antigenic conservation during schistosome development. Two antibodies recognizing determinants absent from the egg nevertheless bound to schistosome antigens in hepatic granulomata; this result suggests that circulating worm antigens were also cleared in these lesions. Fifteen antigens were detected in perivascular spaces 5 weeks after infection, before the appearance of eggs in the liver.