An indirect fluorescent antibody technique was used to detect mouse host antigens and parasite antigens on the surface of Schistosoma mansoni. A rabbit anti-mouse rbc antiserum to detect host antigens, and serum from mice immune to S. mansoni was used to detect parasite antigens. Schistosomula prepared after penetration of isolated mouse skin did not possess host antigens but bound antibody from immune serum (immune antibody); schistosomula recovered from the lungs of mice five days after infection possessed host antigens and failed to bind immune antibody. In contrast, schistosomula recovered from the skin of normal or immune mice three and 20 hours after cercarial penetration, adult worms, and cryostat sections of adult worms, were positive for host antigens but also bound immune antibody. Strong binding of immune antibody only occurred with the cryostat sections. Anti-schistosome antibody can therefore bind to schistosomes in the presence of host antigen. The lung forms of schistosomula however, may have different surface properties as there is no evidence that immune antibody binds to these forms.