The present study was performed to determine whether H-2 matching is required for full cytotoxic T lymphocyte (CTL) responses to allo-H-2 antigens in allogeneic bone marrow chimeric mice. A number of irradiated, bone marrow-reconstituted chimeras constructed from various combinations of marrow cells from B10 H-2 recombinant strains and AKR recipient mice were prepared. Spleen cells obtained from such chimeras and normal control mice were activated in vitro by culturing them with irradiated stimulator cells. It was shown that spleen cells from [4R----AKR], [(4R X 3R)F1----AKR] or [AQR----AKR] chimeras, which were histocompatible on the left hand-side of the H-21 subregion between donor and recipient mice, generated greater CTL activities than those that were seen with spleen cells of [3R----AKR] or [5R----AKR] chimeras, which were histoincompatible in this region. We were unable to demonstrate suppressor cell activity of the spleen cells of [3R----AKR] chimeras cultured with stimulator cells. Although spleen cells from [3R----AKR] chimeras showed substantial proliferative responses to stimulator cells (MLR) and to Con A and LPS, IL2 activities of supernatants from Con A-activated spleen cells (Con A SN) of the chimeras were significantly lower than those of [4R----AKR] or [(4R X 3R)F1----AKR] chimeras. Furthermore, vigorous CTL activities were obtained with either spleen cells or thymocytes from [3R----AKR] chimeras when rat Con A SN was added to the MLR cultures. These observations suggest that the numbers of precursor CTLs in the cells from [3R----AKR] chimeras are at the same level as those of [(4R X 3R)F1----AKR] or normal mice and that the low CTL activities generated by spleen cells of [3R----AKR] chimeras compared to H-2I-matched chimeras are due in large measure to deficiency in IL2 production by the splenic T cells of the [3R----AKR] chimeras.