The H-2 complex and M locus have been defined for two incipient inbred strains of mice carrying the nu mutation. The strains differ at both loci from each other and cells from nu/nu mice of these allogeneic strains have been used to examine the role of T cells in the one-way mixed leukocyte reaction (MLR). A positive MLR requires the presence of both an allogeneic difference, and T cells. Proliferation by T cells is not essential, however, the cultures must contain a cellular component allogeneic to the T cell containing population.