Fischer rats infected with Fasciola hepatica showed significant resistance to Schistosoma mansoni challenge, and vice versa, whereas immunization with 20 Krad-irradiated S. mansoni cercariae failed to protect against F. hepatica challenge, but did protect against homologous challenge. When groups of rats received intraperitoneal implants of newly excysted juvenile flukes, 20- to 22-day-old juveniles, or 8- to 10-week-old flukes, none was significantly protected against S. mansoni challenge, whereas juvenile implants did protect against homologous F. hepatica challenge. In passive transfer experiments in rats, serum from F. hepatica-infected rats or rabbits protected recipients against homologous, but not heterologous challenge, and serum from rats vaccinated with 20 Krad-irradiated S. mansoni cercariae protected recipients against homologous, but not heterologous challenge. These experiments provide evidence that the mechanisms involved in homologous and heterologous resistance are different, the latter lacking immunological specificity. Microsphere injections in F. hepatica-infected rats demonstrated 'shunting' from the portal system to the systemic circulation. If migrating schistosomula are also 'shunted' in Fasciola-infected rats, this, rather than immunologically specific effector mechanisms, might explain their failure to establish in the portal system.