(CBA × C57B1)F1 mice were primed to the hapten NNP on either of the non-cross-reacting carriers fowl γ-globulin (Fγ G) or ovalbumin (OV) and 4 weeks later were given 50 × 106 CBA or (CBA × C57B1)F1 spleen cells. When challenged on the same day with NNP.Fγ G, the two groups of mice undergoing a graft-versus-host reaction (GVHR) showed higher numbers of antibody-producing cells to both NNP and Fγ G determinants. The antibody-forming cells were of host origin, and the effect could be induced by T lymphocytes, not by B. When mice were challenged with NNP.Fγ G at various intervals after the induction of GVHR, it was found that by 8 days the increased response was seen only in mice primed to hapten on the heterologous carrier (NNP.OV). Increased antibody formation was not seen in primed CBA mice injected with (CBA × C57B1)F1 spleen cells, or in normal (CBA × C57B1)F1 mice injected with primed CBA cells. When mice undergoing GVHR were lethally irradiated, they did not increase the response of primed (CBA × C57B1)F1 cells transferred to them. The relevance of these findings to the mechanism of interaction between T and B cells is discussed.