To address whether expression of the relevant MHC molecules on thymic epithelial cells (TEC) is sufficient for positive selection, the fate of CD4+ T cells directed by the DR alpha E beta b molecule was compared between two lines of HLA-DRA transgenic mice, DR alpha-24 expressing DR alpha E beta b on TEC and hematopoietic cells, and DR alpha-30 expressing the molecule on TEC but not on other cell lineages. CD4+ T cells were selected to mature on DR alpha E beta b molecules in both lines deficient for the endogenous MHC class I and class II molecules, whereas CD4+ T cell differentiation in DR alpha-30, as compared with that in DR alpha-24, was severely impaired when these mice were crossed with mice transgenic for the DR alpha E beta b-restricted alpha beta TCR. In contrast, CD4+ T cells expressing the same TCR alpha beta as the transgenes were selected to mature in DR alpha-30 crossed with the single TCR beta transgenic mice. Taken together, these results indicate that the restricted expression of DR alpha E beta b on TEC is sufficient for positive selection in the TCR beta and non-TCR transgenic mice, but the expression on both TEC and hematopoietic cells is required for that in the TCR alpha beta transgenic mice. In the light of the difference in the developmental stage at which alpha beta TCR are expressed between TCR alpha beta and TCR beta transgenic mice, the interaction of the TCR alpha beta with DR alpha E beta b molecules expressed on hematopoietic cells required for positive selection in TCR alpha beta transgenic mice could be mapped to the early stage of T cell differentiation and might mimic 'undefined' early signaling of thymocytes in physiological T cell differentiation.