Mature T cells arise from progenitor cells by a complex and poorly understood process of differentiation in the thymus. Thymocytes can be divided into four major compartments on the basis of surface expression of the murine equivalents of CD8 (Lyt-2) and CD4 (L3T4) (refs 1,2). Functionally mature thymocytes express only CD4 or CD8. The CD4-8- subset contains progenitor cells capable of giving rise to all the phenotypic and functional classes of T cells on adoptive transfer. The function of the major population, the CD4+8+ cells, which carry both the CD4 and CD8 antigens, in thymic differentiation is controversial. It has been variously proposed that they represent terminally differentiated cells which die in the thymus or that they represent an intermediate stage and can differentiate into functionally and phenotypically mature single positive T cells. The CD3-antigen receptor complex is probably important in thymic differentiation. The receptor has two functions: recognition and transmembrane signalling. To help clarify the function of CD4+8+ thymocytes in thymic differentiation, the expression and function of the antigen receptor complex on these cells should be determined. We show here that most CD4+8+ thymocytes express CD3 which functions in transmembrane signalling. The consequences of this signalling differ from those in mature T cells, however, in that the CD4+8+ cells do not secrete IL-2, express IL-2 receptor, or proliferate.