The effects of interferon (IFN) gamma on the course of infection with Strongyloides papillosus in calves were investigated. Calves (N = 7 each) were inoculated with recombinant bovine IFNy or control solution daily from day 0 to day 15 following S. papillosus infection. Treatment with IFN-gamma induced an increase in faecal egg output in the peak stage of infection. The IFNgamma-treated animals harboured more worms, especially more immature worms, in the small intestine than control animals at necropsy on day 17, with no decreases in intestinal mucosal mast cells. Both animal groups had similar small numbers of intestinal worms at necropsy on day 26. All control animals developed peripheral blood eosinophilia on day 7, while five of seven IFN-gamma-treated animals did not. Serum alpha1-acid glycoprotein concentrations increased on day 7 in both animal groups, with higher values in control animals than in IFNgamma-treated animals. Control animals mounted a predominant IgG1 response to S. papillosus from day 10, while IFNgamma-treated animals did from day 22. These data suggested that IFNgamma inhibited some host protective responses to S. papillosus migrating larvae, resulting in an improvement of worm survival after a period when protective responses should be activated during the early stage of infection. The effects of IFNgamma on intestinal worm expulsion should be confirmed by further experiments.