Serological and biochemical studies using monoclonal antibodies have demonstrated that the Ly-15 cell membrane alloantigens are polymorphic sites on the lymphocyte function-associated antigen-1 (LFA-1) molecule. Ly-15.2 and LFA-1 show identical tissue distributions, being present on all thymocytes, lymphocytes, and neutrophils, and flow cytofluorometric analysis indicated identical cell surface expression of these molecules. Identity of Ly-15.2 and LFA-1 was confirmed by immunochemical analysis. The Ly-15.2 and LFA-1 molecules have an identical heterodimeric structure of Mr 180,000 alpha chain and Mr 94,000 beta chain, which coelectrophorese on two-dimensional NaDodSO4/PAGE. Furthermore, anti-Ly-15.2 and anti-LFA-1 antibodies coprecipitate the same molecule from thymocyte lysates, and peptide mapping studies show that the Ly-15.2 and LFA-1 alpha chains are identical, as are the beta chains.