In this report we describe a novel surface molecule, termed CK226, which mediates activation of human T lymphocytes. This molecule was identified by the monoclonal antibody (mAb) CK226 that had been raised against the human leukemic T cell line CEM/K. The CK226 mAb reacted with 10%-35% of human peripheral blood lymphocytes, virtually all monocytes, polymorphonuclear leukocytes and large granular lymphocytes. As demonstrated by two-color cytofluorimetric analysis, the majority of CK226+ lymphocytes were CD3+, whereas a minority coexpressed surface immunoglobulins. Biochemical analysis showed that the surface molecules immunoprecipitated by CK226 mAb from lysates of 125I surface-labeled cloned T lymphocytes were represented by two distinct polypeptides. Their apparent molecular mass was 160 kDa and 80 kDa under nonreducing conditions and 75 kDa and 80 kDa under reducing conditions. CK226 mAb induced proliferation of peripheral blood lymphocytes and interleukin 2 release. Cell proliferation was optimal in the presence of submitogenic doses of phorbol 12-myristate 13-acetate. Finally all T cell clones analyzed for their responsiveness to CK226 mAb released interleukin 2 in the presence of the antibody, regardless of their CD4+ or CD8+ phenotype. Antibody-induced modulation of CD3/T cell receptor molecules inhibited further responses of cloned cells to CK226 mAb. Thus, we conclude that CK226 mAb defines a novel surface molecule which initiates an alternative pathway of T cell activation in humans.