We identified a new human, T-lineage restricted glycoprotein of molecular weight 120 Kd that is expressed primarily in mature T-cell malignancies. The antigen, named TCA-1 (T-cell cytoplasmic antigen), is an intracellular glycoprotein found mainly in the Golgi stacks, although a few cell lines also display surface membrane TCA-1. Many but not all T-cell neoplasms express this antigen. The antigen is absent from neoplastic and normal human tissue outside the T-lymphocyte lineage. TCA-1 was identified by murine monoclonal antibodies produced after immunization of mice with T-cell chronic lymphocytic leukemia cells. The glycoprotein is a monomer containing approximately 4% N- linked carbohydrate with terminal D-galactose residues. Partial amino acid sequence analysis of TCA-1 shows homology with an immunoglobulin heavy chain region, which suggests that TCA-1 may belong to the immunoglobulin supergene family of receptor and adhesion molecules.