This report describes a murine monoclonal IgM antibody, 6E3.C4, induced by retrovirus infection of BALB/ c-H-2b mice which inhibits mitogen stimulation of both mouse and human lymphocytes in vitro. The molecule bound by this antibody appears to be an activation antigen since binding is upregulated by mitogen stimulation. Analysis of the epitope bound by mAb 6E3.C4 revealed that it is associated with a 52-kDa protein with a pI of approximately 5.7 as determined by Western blot analysis. A protein expressing this or a cross-reactive epitope was isolated and determined to be alpha-tubulin by amino acid sequencing. Reactivity with purified alpha-tubulin confirms this identification. These findings suggest a potential role for a cell surface molecule that is either alpha-tubulin or a cross-reactive molecule in the activation processes of T cells.