A new heterodimeric structure, Tp135-145, which can mediate interleukin 2 (IL2) production and Ca2+ mobilization by Jurkat cells is described. This structure was identified by a monoclonal antibody, MX24, on the surface of either T3/TcR+ or T3/TcR- human T cell lines as well as on B cell lines. Biochemical studies showed that antibody MX24 precipitated two polypeptide chains of 135 and 145 kDa, respectively, in lysates from 125I-labeled T cells. After reduction the 135-kDa polypeptide chain shifted to 140 kDa, whereas the molecular mass of the other polypeptide remained unchanged. The apparent molecular masses of the desialylated polypeptides differed by 5 kDa. No common peptide fragments between the two polypeptide chains were found after limited proteolysis by Staphylococcus aureus V8 protease. The expression of Tp135-145 was independent of the expression of the T3/TcR molecular complex. Incubation of Jurkat cells with anti-TcR or anti-T3 monoclonal antibody induced complete modulation only of the T3/TcR complex but not of Tp135-145. Conversely complete modulation of Tp135-145 was observed after incubation of these cells with MX24 antibody. Functional studies showed that anti-Tp135-145 antibody MX24 induced high levels of IL2 production in Jurkat cells. In addition, incubation of these cells with MX24 resulted in Ca2+ mobilization from internal stores. In peripheral blood, Tp135-145 was found to be expressed by 39%-76% of resting T cells in individual donors. Two-color flow microfluorimetry showed that the Tp135-145+ cells were equally distributed on the CD4+ and CD8+ subsets. Incubation of peripheral blood T cells with antibody MX24 resulted in IL2 production and cell proliferation. Taken together these results suggest that Tp135-145 is a novel surface molecule involved in antigen-independent pathway of T cell activation.