Antibody levels were compared in guinea pigs with genetically determined differences in their ability to generate protective immunity against the small-intestine nematode parasite Trichostrongylus colubriformis. Animals with the most effective immune response (high responders) developed significantly higher anti-T. colubriformis IgG 1 antibody titres than low-responder animals. However, there were no significant differences between their IgG 1 antibody responses to a systemically administered protein antigen (ovalbumin). High-titre anti-T. colubriformis serum from high-responder animals did not transfer significant passive protective immunity to low-responder recipients. It is suggested that anti-T. colubriformis IgG 1 antibodies mediate the release of mast-cell and basophil products at the site of infection and thus contribute to the more effective immunity expressed by high-responder animals.