Early stages of T cell development are thought to include a series of coordinated interactions between thymocytes and other cells of the thymus. A monoclonal antibody specific for mouse CD81 was identified that blocked the appearance of alpha beta but nor gamma delta T cells in fetal organ cultures initiated with day 14.5 thymus lobes. In reaggregation cultures with CD81-transfected fibroblasts, CD4(-)CD8(-) thymocytes differentiated into CD4(+)CD8(+) T cells, Thus, interactions between immature thymocytes and stromal cells expressing CD81 are required and may be sufficient to induce early events associated with T cell development.