NZB mice produce a natural thymocytotoxic autoantibody (NTA) capable of specifically injuring thymocytes and T cells. NTA-reactive antigen (NTA-A) shows a different density distribution among T cells, and partial killing with NTA and complement can eliminate T cells bearing NTA-A in high density. Thy-1 antigen is similar to NTA-A in this respect. To determine the effects of NTA and anti-Thy-1 on distinct functional subsets of T cells, Con A-induced suppressor T cell (Con A-Ts) activity against the allogeneic mixed lymphocyte reaction (MLR), responding T cell (TMLR) activity in the allogeneic MLR, and Con A-induced cytotoxic T cell (Con A-Tc) activity were examined simultaneously in BALB/c spleen cells before and after partial elimination of NTA- and anti-Thy-1-sensitive T cells. Treatment with NTA and complement resulted in a marked reduction in Con A-Ts activity, a significant increase in TMLR-activity and a slight and inconstant decrease in Con A-Tc activity. Since Con A-generated T's were much less sensitive to NTA, the NTA-sensitive T cells involved in Con A-Ts activity appear to be precursors or promoters of the Con A-Ts. In contrast, the precursors of Con A-Tc seem to relatively resistant to NTA. The increase in TMLR activity caused by NTA suggests the possibility that NTA is less cytotoxic for TMLR and cytotoxic for some suppressor T cells in allogeneic MLR. The monoclonal anti-Thy-1 antibody showed no such preferential cytotoxic effects on the three T cell functions. The NTA-sensitive T cells, in contrast to anti-Thy-1-sensitive T cells, were reduced gradually during Con A stimulation. All these findings indicate that NTA-A not only differs from Thy-1 antigen but that it appears to be a unique T cell antigen.