AbstractB2A2‐CD4‐8‐cells represent a rare subpopulation of thymocytes normally comprising 0.5% of the total adult thymus. These cells are known to express CD3‐associated T cell receptor (TcR) α/ß molecules. In the present study we have examined the functional capacity of α/ß heterodimers on B2A2‐CD4‐8‐cells. In the presence of monoclonal antibody (mAb) specific for either murine CD3 or TcR expressing the Vβ8‐encoded ß chain (F23.1), B2A2‐CD4‐8‐cells proliferated. Such proliferation was blocked by mAb to interleukin 2 receptor (IL 2R), suggesting an autocrine mechanism involving IL 2 production and subsequent utilization. IL 2 and also IL 3 production by mAb‐stimulated B2A2‐CD4‐8‐cells was directly confirmed. Furthermore, a panel of B2A2‐CD4‐8‐clones were derived to assess the role of the TcR in cytolysis. Many clones were isolated which killed Fc receptor‐bearing P815 target cells only in the presence of F23.1 mAb. Finally,in vivotreatment of neonatal mice with F23.1 mAb resulted in a marked reduction of Vβ8 B2A2‐CD4‐8‐thymocytes. Collectively, these results indicate that the TcR α/ß complex on CD4‐CD8‐B2A2‐cells is fully capable of transducing signals that lead to proliferation, lymphokine production and cytolysisin vitro, as well