Anti-mouse T3 complex CD3 monoclonal antibody is specific for a 25 kDa protein component ( -T3) of the antigen specific T cell receptor on all mouse strains tested. The -T3 protein has been shown to be non-covalently associated with the cell surface heterodimer of the CD3 associated complex. This monoclonal antibody reacts with all mature T cells and can both activate and inhibit T cell function (1). This fact identifies -T3 as a cell surface protein involved in the transduction of activation signals. All peripheral T cells express this determinant, however, B cells and bone marrow cells have proven to be negative. Although the expression of this particular epitope on peripheral T cells is uniformly high, staining of thymocytes reveals distinct subpopulations of cells differing in the level of expression of this marker. This antibody will prove useful in studying the role of various components of the TCR complex in T cell activation and development, and will allow for the development of an animal model in which to investigate the immunoregulatory effects of in vivo administration of anti-CD3 antibodies, an area of obvious clinical