The effect of the natural thymocytotoxic autoantibody (NTA) of NZB mice on the functional T cell subsets was studied in an in vitro secondary antibody response. Dinitrophenylated keyhole limpet hemocyanin (DNP-KLH)-primed BALB/c spleen cells were treated with appropriate dilutions of NTA and complement, and then stimulated with DNP-KLH. Whereas the cytotoxic treatment of DNP-KLH-primed spleen cells with a low dilution of NTA and complement slightly suppressed the anti-DNP antibody response, the treatment at a higher dilution resulted in a significant enhancement of the antibody response. The enhancement was found due to the selective elimination of a part of T cells including suppressor T cells, which were more sensitive to NTA than helper T cells. This was confirmed by an experiment in which KLH-primed spleen cells were treated with appropriate dilutions of NTA and complement and then co-cultured with DNP-KLH-primed B cells resulting in a marked enhancement of the anti-DNP antibody response. The treatment of KLH-primed spleen cells with NTA in the same manner resulted in the loss of activity of the extractable antigen-specific suppressive T cell factor (TsF). Both antigen-specific helper T cells and memory B cells were found to be highly resistant to the cytotoxicity of NTA. Fluorescence-activated cell sorter (FACS) analyses revealed that the cytotoxic treatment of splenic T cells with NTA with an appropriate dilution selectively eliminated the T cells bearing a high density of NTA-reactive antigen. These results suggest that NTA is responsible for the selective loss of the suppressor T cell function that appears early in life in New Zealand mice and plays, at least in part, a causative role in the development of autoimmune diseases.