CD2 is a T lymphocyte glycoprotein that functions in adhesion of T lymphocytes and also as a putative receptor for activation signals. Functional data suggest that LFA-3, a widely distributed cell surface glycoprotein, may be the biological ligand of CD2. We have purified LFA-3 from human erythrocytes and characterized the purified protein functionally. LFA-3 bound specifically to CD2+ cells, and this binding was inhibited by CD2 mAb. Conversely, purified LFA-3 inhibited binding of CD2 mAb to cells, and the concentration required for this effect suggests that LFA-3 half-saturated CD2 at 1-5 nM LFA-3. Purified LFA-3 inhibited rosetting of human and sheep erythrocytes with CD2+ T lymphoma cells and T lymphocytes, and mediated aggregation of a CD2+ T lymphoma cell line. Purified LFA-3 reconstituted into planar membranes mediated efficient CD2-dependent adhesion of T lymphoblasts. These data demonstrate that LFA-3 is a ligand for CD2 and that LFA-3 can mediate T lymphocyte adhesion.