The experiments in this report, together with previous studies, demonstrate that the light chains of DR antigens are composed of four domains: two large extracellular domains (each with a disulfide loop), a short hydrophobic membrane-binding region, and a short hydrophilic carboxy-terminal domain. The amino-terminal extracellular domain is glycosylated and polymorphic and has no discernible sequence homology to immunoglobulin, whereas the relatively conserved carboxy-terminal extracellular domain bears striking homology to immunoglobulin. One chymotryptic and two tryptic cleavage sites lie in the second extracellular domain of the light chain of all native DR antigens. These three sites are found in loops at the same end of an immunoglobulin-like domain. The light chain of DC1 antigen lacks one of the tryptic cleavage sites; the DR heavy chain, the class I heavy chain, and beta 2 microglobulin lack all three proteolytic sites. Proteolysis of murine la antigens with chymotrypsin and trypsin generates similar fragments (I-E-like DR, I-A-like DC1) suggesting these cleavage sites are a general feature of class II antigen light chains.