F4 receptor-positive (F4R+) and F4 receptor-negative (F4R-) pigs were orally vaccinated with purified F4 fimbriae of enterotoxigenic Escherichia coli (ETEC). Serum immunoglobulin G (IgG) and IgA responses were readily detected in F4R+ animals, whereas immune responses were not detected in F4R- animals. Even after a subsequent oral infection with virulent F4(+) ETEC and a booster immunization with F4, the F4R- animals remained F4 seronegative whereas the unvaccinated F4R+ pigs exhibited clear IgA and IgG responses. These results clearly demonstrate that F4Rs are a prerequisite for an immune response following oral immunization. Furthermore, indications that oral F4 vaccination can induce mucosal protection were obtained, since the experimental ETEC infection did not induce a systemic booster response or fecal ETEC excretion in orally vaccinated F4R+ pigs, in contrast to the clear immune response and ETEC excretion of unvaccinated F4R+ animals. F4-specific IgA antibodies could be found in the feces of the vaccinated F4R+ pigs. They are secreted at the intestinal mucosal surface and appear to prevent ETEC infection. The F4R-dependent induction of a mucosal immune response can be used as a model to better understand mucosal immunization and mucosal immune responses and can contribute to the development of oral vaccines in veterinary as well as in human medicine.