A new target antigen for unrestricted killing was defined by NZB T lymphocytes which were immunized and restimulated with H-2-identical BALB/c spleen cells. These effector cells killed nearly all target cells tested, irrespective of their H-2 type, but did not kill NZB target cells. The response was shown to have three major components: unrestricted killing specific for Qed-1b, H-2d-restricted killing specific for minor histocompatibility antigens, and unrestricted killing specific for a new antigen, Mta. Mta is present on normal and mitogen-stimulated T and B lymphocytes and on several tumor lines. It was found on cells from 26 mouse strains tested, including two substrains of NZB, representing 9 different H-2 types and 14 different non-H-2 backgrounds. Analysis of the NX8 recombinant inbred lines (derived from Mta-NZB/Icr and Mta+C58/J parents) suggested that Mta is maternally transmitted. This was confirmed by typing of reciprocal F1 hybrids and backcrosses between positive and negative strains: Mta+ females bear Mta+ offspring and Mta- females Mta- offspring, irrespective of the phenotype of the males.