Serum antibody responses of mice exposed to Philippine isolates of Schistosoma japonicum have been analysed by immunoprecipitation of exogenously radiolabelled antigens extracted from adult worms. Attention was focused on labelled protein antigens differentially recognized by sera of mice that differ genetically in their resistance status. Mice of the inbred strain 129/J can show high level resistance to first or repeated infection with S. japonicum. Even after six percutaneous administrations of 25 cercariae, approximately 50% of 129/J mice remain healthy with no or very few worms present in the portal system. Sera from 129/J mice exposed to S. japonicum consistently and differentially recognise an antigen of adult worms of mol. wt. 26,000. This antigen, termed Sj26, is not immunoprecipitated from S. mansoni adult worms by sera from resistant 129/J mice. Serum antibodies to Sj26 are present in at least some patients with a history of schistosomiasis japonica. Whether immune responses to Sj26 are involved directly in expression of resistance to S. japonicum remains to be determined. However, this antigen produced by cloned DNA in expression vectors, or isolated from adult worms, is an obvious candidate to be tested for vaccination efficacy in mice.