Graft-versus-host reaction induced in 6-weeks-old (Balb/c X AKR)F1 hybrid mice by the injection of parental (Balb/c) spleen cells caused the allogeneic effect, i.e. the stimulation of host IgM and IgG antibody forming cells (PFC) to SRBC and LPS. Parental lymphocytes in vivo allosensitized to AKR antigens were used to find out if the alloreactive T lymphocytes mediate the allogeneic effect. It was found that these cells influenced differently the humoral response to the antigens. The stimulation of the response to LPS was not changed in comparison with that produced by normal parental cells. On the 5th day of immunization with SRBC the stimulation of IgM PFC was stronger than IgG PFC. On the 10th day, the stimulation of IgM and IgG PFC was decreased. Moreover, it was observed that when parental splenocytes normal or allosensitized were incubated and centrifuged on allogeneic fibroblast monolayer the ability of nonadherent cells to mediate allogeneic effect was decreased or completely disappeared.