T-cell cytotoxicity of NZV mice was tested after in vitro sensitization against a group of H-2 identical strains (BALB/c, B10.D2, DBA/2, HW19). A highly significant and unexpected unidirectional cell-mediated lympholysis (CML) reaction by the sensitized NZB effector cells on these targets was found. After sensitization in vitro with stimulator cells of one H-2d strain, NZB effector cells (H-2d) lysed all other H-2d targets and to a lesser degree, some non-H-2d targets (C57BL/10, DBA/1, B10.Q, CBA, B10.S, A.SW). NZB targets were not lysed. Differences in the major histocompatibility region between NZB and other H-2d strains could be excluded as a possible explanation for the observed reaction of NZB (H-2d) against other H-2d strains. These results consequently represent the first description of a primary in vitro CML directed against determinants not coded for in the major histocompatibility complex. The responsible effector cells are demonstrated to be T cells. The CML of NZB against H-2 identiical targets appears best explained by a reaction against minor histocompatibility antigens. This, and the observed cross-reactions, would indicate that the cytotoxic T-cell system in NZB mice is not subjected to restrictions found in all normal mouse strains tested until now under similar conditions. It is suggested that this hyperreactivity is related to the autoimmune responsiveness of the NZB strain.