Recently the human T cell erythrocyte receptor CD2 has been shown to bind human erythrocytes through LFA-3, a heavily glycosylated surface protein of broad tissue distribution. CD2-LFA-3 interactions are important for cytolytic conjugate formation, for thymocyte adhesion, and for T cell activation. A complementary DNA clone encoding LFA-3 was isolated using a complementary DNA clone encoding LFA-3 was isolated using a novel transient expression system of mouse cells. The cDNA encodes a phospholipid-linked membrane protein whose extracellular domain shares significant homology with CD2. As CD2 is homologous with the neural cell adhesion molecule NCAM in immunoglobulin-like domains, cellular adhesion molecules in both neural and lymphoid tissues could have a common ancestor.