We compared the reaction in immunoblots of sera obtained from patients with parasitologically proven S. mansoni infections, with a suspected history of schistosomiasis infection, or with unrelated parasitic diseases. Several polypeptides from adult S. mansoni reacted with the schistosomiasis patients' sera in a heterogeneous manner. However, a component of approximately 31 kilo daltons (kD) reacted with all schistosomiasis sera and with several sera of suspected schistosomiasis cases. No reaction was ever observed with sera of patients harbouring other parasites. Thus, the polypeptide has potential diagnostic value. The use of sera of patients with recent infections demonstrated that: the earliest time of antibody formation against the 31 kD component was approximately 40 days post infection, the reaction with this polypeptide in immunoblots was exceptionally strong and antibodies directed against other schistosome proteins were barely detectable at this time. Identical results were obtained with sera of experimentally infected mice. The 31 kD component was present in parasites of either sex. It was apparently not a glycoprotein. Evidence suggests that the 31 kD polypeptide may originate from the schistosome gut.