The effect of the graft-versus-host reaction in response to minor histocompatibility antigens on the antibody response to a T-dependent antigen was studied in four strains of mice. Lethally irradiated mice were transplanted with bone marrow plus graded numbers of spleen cells from H-2-compatible donors. Recipients of syngeneic bone marrow transplants and recipients of allogeneic bone marrow depleted of T cells made normal antibody responses to bacteriophage øχ 174 when immunized on day 28 (primary) and day 56 (secondary) after marrow transplantation. Recipients of allogeneic bone marrow plus spleen cells made only small amounts of specific antibody and failed to make IgG antibody after secondary immunization. The pattern of the depressed antibody response suggests that the primary mechanism of immune dysfunction in mice with the minor antigen GVHR is a lack of T helper cell function.