We describe a miniaturized cell-mediated lympholysis assay for measuring the activity of cytotoxic lymphocytes generated in mouse mixed leukocyte cultures (MLC), using either serum-free or mouse serum-supplemented media. Mixed reactions of strain combinations possessing various genetic differences at H-2 produced cytotoxic lymphocytes with cytolytic activity against cells of the donor used in the sensitization phase. Results suggest that both serologically defined and lymphocyte-defined determinants can act as effective target antigens on target cells. In contrast, no cytotoxic lymphocytes were generated against products of non-H-2 loci--M-locus, theta, or other non-H-2 antigens--that induce MLC activation.