Antigens sharing determinants with a circulating schistosomal antigen (CSA) haven been demonstrated in the digestive tract of cercariae, skin penetrating and 5-day schistosomula and adult worms, but not in the eggs of Schistosoma mansoni, by using indirect fluorescence and anti-CSA prepared in rabbits. IgM antibodies in the serum of patients with hepatosplenic schistosomiasis showed a similar pattern of fluorescence as anti-SA antibodies, but also bound to mature eggs. IgG antibodies gave a difuse staining of parasitic structures and IgE antibodies attached to eggs and schistosomulum tegument. IgA antibodies against S. mansoni were not detected in the sera in infected patients. These findings may explain the appearance of CSA before the development of adult worms in experimental infections, and also the presence of antibodies that bind to worm intestinal structures in sera from early human infections. The potential significance of CSA antibodies in schistosomal glomeruolopathy and in resistence to re-infection seems apparent.