Lymphocyte activation induces or increases the expression of several surface structures, some of which are directly involved in cell growth such as receptors for IL2 or transferrin. To identify new structures characteristic of activated lymphocytes, we developed a series of mAbs against functionally defined human T-cell clones or the NK-mediating cell line YT2C2. In this study, we report the isolation of an mAb termed AY19 recognizing, at the cell surface, an 85-kD glycoprotein whose expression is restricted to T-lymphocyte clones, leukemic T-cell lines, and T-ALL peripheral blood cells. Biochemical studies, as well as phenotypic analysis, revealed that this structure is different from all previously identified molecules on the cell surface of lymphocytes. Furthermore, functional studies showed that triggering of this 85-kD structure on cloned T-lymphocytes through the AY19 epitope led to an inhibition of CD3-induced proliferation. These findings suggest that the AY19 mAb defines a T-cell antigen expressed mainly on long-term dividing lympocytes and, therefore, its putative ligand might play a role in the regulation of T-lymphocyte growth induced by CD3-TcR stimulation.