Oral administration of antigen has been shown to be effective for both positive and negative modulation of immune responses. In the present study we characterized changes in the reactivity of the immune system after oral immunization with allogeneic spleen cells. Mice were orally immunized for 10 consecutive days with fresh allogeneic spleen cells, and the phenotype, proliferative response, cytotoxic activity and cytokine production profile of recipient spleen cells were assessed 1 or 7 days after the last immunization dose. Although no significant changes in the proportion of CD4+, CD8+ or CD25+ cells were observed in the spleen of orally immunized mice, significant activation of alloreactivity in spleen cells was found. Cells from orally immunized mice exhibited enhanced proliferation and cytotoxic activity after stimulation with specific allogeneic cells in vitro, and produced considerably higher concentrations of interferon-gamma (IFN-gamma) and significantly less interleukin (IL)-4 than did cells from control mice. The production of IL-2 was essentially unchanged and that of IL-10 was only slightly increased. The systemic allosensitization induced by oral immunization was demonstrated in vivo by increased resistance to the growth of allogeneic tumours induced by subcutaneous inoculation of high doses of tumour cells. In addition, orthotopic corneal allografts in orally immunized recipients were rejected more rapidly (in a second-set manner) than in control, untreated recipients. These data show that oral immunization with allogeneic cells modulates individual components of the immune response and that specific transplantation immunity, rather than tolerance, is induced in the treated recipients.