The TCR is a multisubunit complex composed of the clonotypic alpha/beta disulfide-linked heterodimer and noncovalently linked invariant CD3 gamma epsilon and CD3 delta epsilon and TCR zeta chains. Recent studies demonstrate that the surface expression of CD3 components can occur independently of the clonotypic TCR complexes in both thymocytes and splenic T cells. In this study, we report that free noncovalently associated TCR alpha beta heterodimers that exist independently of CD3 and TCR zeta chains are expressed on the cell surface of immature thymocytes and peripheral T cells, but not of T cell lines and T cell hybridomas. This suggests that the regulation of surface expression of TCR alpha beta heterodimers differs between primary T cells and T cell lines or T cell hybridomas. The isolation and biochemical characterization of surface clonotype-independent CD3 complexes and free membrane-associated TCR alpha beta complexes may provide a structural basis for the quantitative difference in amount of T cell proliferation stimulated by anti-CD3 epsilon and anti-TCR beta.