Monoclonal antibodies were used in an inhibition enzyme-linked immunosorbent assay (IELISA) to detect a variety of schistosome derived antigens. Preparations were obtained from various stages of Schistosoma mansoni and from the eggs of S. japonicum. Using appropriate titers of monoclonal antibodies it was possible to detect less than 0.01 micrograms/ml of schistosome antigens. The sensitivity of IELISA was dependent upon the type and concentration of the monoclonal antibody used, as well as upon the source of the antigens. Specificity studies showed that some of the monoclonal antibodies recognized species specific antigenic determinants, while others reacted against genus specific antigens. Furthermore, certain antibodies interacted with antigens which were neither genus or species specific. Fractions of S. mansoni and S. japonicum egg antigen extracts, which have been previously considered to be relatively pure, were compared using certain monoclonal antibodies. The results indicate that these fractions are very heterogenous with regard to the unique antigenic specificities. For example, most antigenic determinants in crude soluble egg antigen are not retained on concanavalin-A lectin affinity columns and major serologic antigens have more than one determinant with different distributions. The expression of these antigenic determinants appears to be a function of both the concentration of the specific antigen and the mode of expression of the moiety within the antigenic matrix.