The cytotoxic T cell response in outbred mice infected with lymphocytic choriomeningitis virus (LCMV) is strain specific. The same is true for adoptive transfer of fatal LCM disease. The response of individuals within an outbred strain is completely cross-reactive, as shown by using immune lymphocytes and virus-infected macrophage targets from individual mice. Reciprocal exclusion of cytotoxic T cell activity between inbred and outbred mouse strains is the rule, the exception being one strain (H) known to have some C57BL ancestry. Immune T cells from one of 7 H mice specifically lysed LCMV-infected C57BL macrophages. Experiments with inbred mice have shown that only one allele need be shared at either the H-2K or H-2D locus for cytotoxic T cell activity to be manifest. Adoptive transfer protocols may thus be considered in outbred situations, providing that T cells are effective before allograft rejection occurs. Also, the LCMV cytotoxic T cell assay may be useful for determining the degree of H-2 variability in wild mouse populations, as novel H-2 types can be detected and mice need not be congenic.