Papain solubilization enables 2 molecular models to be obtained according to which SLA specificity is studied. The SLA 1 antigen in the soluble state is a molecule having an apparent MW of about 40,000 daltons. It is composed of 2 polypeptide subunits with MW of 28,000 and 12,000 daltons. The MW of papain-solubilized SLA 7 antigen is twice that of the SLA 1 antigen. Partial dissociation of the SLA 7 antigen produces 42,000-dalton structures, while total dissociation yields polypeptides having MW similar to those polypeptides constituting the SLA 1 antigen. All of these facts indicate that the soluble form of the SLA 7 antigen is a tetracatenary structure including 2 of each polypeptide. The SLA L chain is shown to be a .beta.2-microglobin which cross-reacts with human .beta.2-microglobulin.