A self-I-A reactive T cell clone has been isolated that proliferates in the presence of irradiated syngeneic spleen cells in mouse serum. The cells directly stimulate approximately every hundredth B cell, which includes small B cells, into both proliferation and antibody formation in the absence of added antigen. There is evidence to suggest that cells of this type are generally present in murine T cell populations. The cells may recognize self-I-A as such. We failed to obtain evidence, but cannot exclude that they recognize an antibody idiotype in addition. The clone also augments antigen-specific antibody responses in vivo and in vitro in the presence of antigen. In the in vitro response to a T-independent TI-2 antigen, the T cells in co-culture with B cells induced specifically the production of IgG1 antibodies, thus mimicking the function of major histocompatibility complex-restricted, antigen-specific helper T cells. However, IgG1 production was not observed when splenic T cells were added to this system. Thus, self-I-A reactive T cells, generally inactive in the system, may locally help B cell responses to antigens which the T cells do not themselves recognize. In such instances one might attribute to the T cells antigen or idiotype-binding specificities which the cells in reality do not display.