Abstract: Outbred Kunming mice infected with cercariae of Schistosoma japonicum were killed at definite intervals after infection. The serum of each mouse was assayed by indirect immunofluorescent antibody test for IgG against the surface antigen of cercaria. The worms in the same mouse were collected and examined for their stages of development. It was found that sera from mice with bisexual infections showed earlier appearance of IgG against cercaria and higher level as well than those from mice with unisexual male infections. Mouse anti-cercaria IgG could be absorbed with macerated adult worms or supernatants from adult worm cultures, but not with immature eggs nor cultures of mature eggs. Anti-cercaria IgG collected from infected mice showed strong immunofluorescence on the surface of both male and female adult worms, but not on the digestive tract not uterus. The intensity of fluorescence on the surface of adult worms by anti-cercaria sera diminished significantly after the sera had been repeatedly absorbed with cercariae. It indicates that the body wall of adult S.. japonicum and its turnover products can elicit the mouse antibodies against antigen (s) on the surface of cercaria, and that there are antigens common to adult worm and cercaria of S. japonicum.