Three IgM monoclonal antibodies, M22G11P, M7B7 and M22B3G, which reacted with the surface of Schistosoma mansoni schistosomula in an indirect fluorescent antibody assay, were found to recognize a polysaccharide-containing egg antigen previously designated K3. The monoclonal antibodies and a monospecific rabbit anti-K3 serum also recognized a crossreacting antigen in a crude cercarial antigen preparation. In an eosinophil-mediated schistosomulum killing assay, all three monoclonal antibodies significantly inhibited the level of killing produced by human infection serum. An IgG3 monoclonal antibody, M22C1C, which also recognized the egg antigen K3, did not inhibit eosinophil-mediated killing. However, when lower concentrations of human serum were used in the assay, this monoclonal antibody significantly enhanced the level of killing, despite having no capacity to induce eosinophil-mediated damage in the absence of human infection serum. On the basis of these and other results we suggest the possibility that antibodies to S. mansoni egg antigens which cross-react with the surface of the early post-penetration schistosomulum may influence the effective expression of antibody-dependent, eosinophil-mediated effector mechanisms in human infections.