In the thymus, precursor T cells differentiate recognition structures for self that are specific for the H-2K, D, and I markers expressed by the thymic epithelium. Thus recognition of self-H-2 differentiates independently of the T cells H-2 type and independently of recognition of nonself antigen X. This is readily compatible with dual recognition by T cells but does not formally exclude a single recognition model. These conclusions derive from experiments with bone marrow and thymic chimeras. Irradiated mice reconstituted with bone marrow to form chimeras of (A X B)F1 leads to A type generate virus-specific cytotoxic T cells for infected targets A only. Therefore, the H-2 type of the host determines the H-2-restricted activity of killer T cells alone. In contrast, chimeras made by reconstituting irradiated A mice with adult spleen cells of (A X B)F1 origin generate virus-specific cytotoxic activity for infected A and B targets, suggesting that mature T cells do not change their self-specificity readily. (A X B)F1 leads to (A X C)F1 and (KAIA/DC) leads to (KAIA/DB) irradiation bone marrow chimeras responded against infected A but not B or C targets. This suggests that cytotoxicity is not generated against DC because it is abscent from the host's thymus epithelium and not against DB because it is not expressed by the reconstituting lymphoreticular system. (KBIB/DA) leads to (KCIC/DA) K, I incompatible, or completely H-2 incompatible A leads to B chimeras fail to generate any measurable virus specific cytotoxicity, indicating the necessity for I-specific helper T cells for the generation of killer T cells. Finally adult thymectomized, irradiated and bone marrow reconstituted (A X B)F1 mice, transplanted with an irradiated thymus of A origin, generate virus-specific cytotoxic T cells specific for infected A targets but not for B targets; this result formally demonstrates the crucial role of thymic epithelial cells in the differentiation of anti-self-H-2 specificities of T cells.