Lymphatic filariasis caused by the parasitic nematode, Brugia malayi, is a chronic human disease immunologically characterized by stimulation of Th2 cells and reduced antigen-specific T cell responses. Single stage intra-peritoneal infections with infective larvae (L3) or adult nematodes induce Th2 cells, while the microfilarial stage (Mf) stimulates IFN-γ and Mf-specific IgG1, IgG2a, IgG2b, IgG3 and IgM, but not IgE. To investigate whether IFN-γ is elicited by live Mf in their natural site of infection, mice were infected intravenously. Intravenous infection had a striking effect on the response to Mf and high levels of IgE were induced even in the presence of IFN-γ. Indeed IgE levels to Mf increased markedly with the number of immunizations, higher doses of Mf and prolonged exposure to Mf suggesting that under conditions of chronic antigen exposure, typical of human disease, Mf will stimulate high levels of IgE. The ability of Mf-induced IFN-γ to modulate or regulate a pre-existing Th2 response, was investigated by infecting mice initially with adult male worms to induce a Th2 response, followed 14 days later by infection with Mf. Although Mf stimulated IFN-γ in the presence of male adults, the antibody isotypes elicited did not reflect IFN-γ induction and IgG1and IgE dominated the response. Although it cannot be discounted that IFN-γ induction by Mf may act locally as an inflammatory mediator or modulator of Th2 cells, these data suggest that Mf-stimulated IFN-γ does not have a profound effect overall on progression of the Th2-dominated immune response to filarial infection.