To assess the effect of ivermectin on immune function (antibody production), male CD-1 mice were inoculated with an antigen the day after SC administration of ivermectin (0.2 mg/kg of body weight or 20 mg/kg). Responses were evaluated 5 days after inoculation of the antigen. Antibody production against sheep RBC, a T lymphocyte-, macrophage-dependent response, was enhanced by ivermectin treatment (P = 0.00049). In contrast, antibody production against dinitrophenyl-Ficoll, a T lymphocyte-independent, macrophage-dependent response, was not altered by ivermectin treatment. Results indicate that the immunostimulatory properties of ivermectin are associated with altered function of T lymphocytes, in particular, T-helper lymphocytes. The immunomodulating effects of ivermectin may provide an alternative approach for treatment of disease problems involving immunosuppression.