Eosinophils from the peritoneal cavity of normal rats, in the presence of fresh normal rat serum (NRS), adhered to schistosomula of Schistosoma mansoni in vitro and killed the majority of parasites within 18 h. The reaction differed from the previously described antibody-mediated eosinophil adherence to schistosomula which occurs in heat-inactivated immune rat serum (IRS) and where adherence is mediated through Fc receptors. Adherence of eosinophils in fresh NRS was shown to be due to the activation of complement at the schistosomular surface by the alternative pathway, and it was effected through C3 receptors. The ability of eosinophils to kill in Fc-mediated adherence. This enhancement of killer activity may be due to the generation by complement activation of eosinophil chemotactic factors which increase the concentration of cells at the target surface. It is suggested that eosinophil adherence mediated through complement activation could be the principla mechanism of destroying schistosomula in the host.