Abstract Transgenic expression of the beta-chain of T cell antigen-receptor (TCR) is known to induce the generation of CD4+ CD8+ thymocytes in the immunodeficient scid mouse, in which thymocyte development is otherwise arrested at CD4- CD8- cells. It is not clear, however, whether or not the thymocyte development is controlled by ligand engagement of the TCR-beta complex on the cell surface. In the present study, we have examined how the engagement by Ab of the TCR-beta complex expressed on the TCR-beta-transgenic scid fetal thymocytes can regulate the generation of CD4+ CD8+ thymocytes. Organ cultures of CD4- CD8- day 14 fetal thymocytes from the TCR-beta-transgenic scid mice resulted in the generation of CD4- CD8+ and then CD4+ CD8+ cells. The initial step from CD40- CD8- cells to CD4- CD8+ cells was enhanced by the addition of anti-TCR-beta Ab, whereas the subsequent step from CD4- CD8+ cells to CD4+ CD8+ cells was markedly inhibited by anti-TCR-beta Ab. These results indicate that ligand engagement of the TCR-beta complex can positively and negatively regulate the early thymocyte development. Moreover, the finding that engagement of TCR-beta complex inhibits the generation of CD4+ CD8+ cells suggests that the induction of CD4+ CD8+ thymocytes by the TCR-beta transgene is not an immediate consequence of cell-surface engagement of the TCR-beta complex but requires liberation from the continued TCR-beta signaling.