A simple method for producing monospecific rabbit antisera, applied originally to the constituents of human serum (Goudie et al. 1966), has been adapted for use with Schistosoma mansoni egg antigens. Replicate isolated immunoprecipitin arcs resulting from the immunoelectrophoretic reaction between an egg antigen and a polyspecific antiserum were excised, washed extensively to remove non-precipitated contaminants, homogenized, and emulsified with complete Freund's adjuvant. Rabbits were given weekly subcutaneous injections of the emulsion in multiple sites, and a monospecific precipitating antibody against the respective immunizing antigen generally resulted within 6 weeks of the commencement of immunization. Antisera raised in this manner against S. mansoni egg antigens omega 1, alpha 1 and kappa 5 have been used to characterize the antigen with respect to their stage- and species-specificity. After immunoabsorption to remove background activity, the sera could be used to detect unequivocally the respective antigens in crude egg homogenate that had been subjected to SDS-PAGE and electrotransfer to nitrocellulose paper.