The effects of sera from S. mansoni-infected mice together with eosinophil- and macrophage-rich peritoneal exudate cells (PEC) on the schistosomula were examined in vitro. Adherence to and killing of the parasite by PEC is mediated through a complement-independent opsonizing IgG. Destruction of schistosomula was assessed morphologically and by evaluating their infectivity to normal mice. By using ferritin-labeled anti-mouse immunoglobulin, the antibody was localized on the surface of the schistosomula and at the cell-organism areas of contact. The serum antibody activity appears with the onset of egg production, does not correlate well with the duration of infection, and is relatively specific among schistosome species. Eosinophils and macrophages were demonstrated by electron microscopy to adhere firmly to the surface of the organisms; the latter cells were shown invading the schistosomula. Furthermore, depletion of eosinophils by prior incubation of PEC with monospecific anti-eosinophil serum blocked the early adherence phase, but a substantial part of PEC ability to kill the schistosomula was retained.