Parental spleen cells injected into F1 hybrid mice stimulated the 7S and, to a lesser degree, 19S primary antibody response to thymus-dependent and -independent antigens. The stimulatory effect on 7S antibody formation as compared to mice, injected with syngeneic spleen cells, was exerted by parental spleen cells injected after, as well as prior to antigenic stimulation. Large amounts of parental spleen cells inhibited the antibody response in comparison to the response obtained after injection of syngeneic cells. Allogeneic cells augmented the 7S and 19S antibody response to very small amounts of antigen. The effect of parental cells on host antibody formation could be abolished by treating the cells with anti-Thy 1 serum and complement which confirms that the stimulatory effect was mediated by T cells. The results indicate that 7S antibody formation is more dependent on non-antigen-specific influences from T cells than is 19S antibody production and are compatible with the hypothesis that activated T cells release substances which stimulate or inhibit antibody formation. These substances do not appear to be responsible for the immunologically specific ‘thymic helper’ effect.