Summary. Mice of the strain 129/J bred at this Institute (WEHI 129/J) were exposed once or repeatedly to cercariae of Schistosoma mansoni and worm burdens determined. In keeping with reports of S. japonicum infection in these mice, a high level of resistance was recorded. Radio-labelled extracted antigens of S. mansoni adult worms were reacted with sera from exposed 129/J mice and infected BALB/c mice. Immunoprecipitated antigens preferentially recognized by antibodies in 129/J mice include proteins of Mr 99,000 and 21,000. Whether responses to these antigens are a cause or a consequence of resistance remains to be established.