Interferon-γ (IFN-γ) receptor knock-out (IFN-γR−/−) mice were used to analyse the role of IFN-γ in mucosal immune responses following oral immunization. We found that the IFN-γR−/− mice demonstrated 50% reduced spot-forming cell (SFC) responses in the gut lamina propria and spleen after oral immunization with keyhole limpet haemocyanin (KLH) plus cholera toxin (CT) adjuvant. The IFN-γR−/− mice exhibited 10-fold reduced total serum KLH-specific antibody levels compared with wild-type mice after oral immunization, while after intravenous immunization, no such difference was seen, suggesting a selective impairment of mucosal immune responses. Moreover, oral immunizations resulted in impaired interleukin-4 (IL-4), IL-10 and IFN-γ production by spleen T cells from IFN-γR−/− mice, indicating that no reciprocal up-regulation of Th2-activities had occurred despite the lack of IFN-γR function. No reduction in Th1 or Th2 cytokines was observed following systemic immunizations. Despite potentially strong modulating effects of IFN-γ on epithelial cell IgA transcytosis and electrolyte barrier functions, CT-immunized IFN-γR−/− mice demonstrated unaltered protection against CT in ligated intestinal loops together with normal anti-CT IgA and total IgA levels in gut lavage. Oral feeding with KLH followed by parenteral immunization resulted in strongly suppressed SFC numbers and reduced cell-mediated immunity in both wild-type and IFNγR−/− mice. CT-adjuvant abrogated induction of oral tolerance in both IFN-γR−/− and wild-type mice. Collectively, our data argue that the two major response patterns induced by oral administration of protein antigen, i.e. active IgA immunity and oral tolerance, are differently regulated. Thus, IFN-γR−/− mice have impaired mucosal immune responses while induction of oral tolerance appears to be unaffected by the lack of IFN-γ functions.