Rabbit heteroantisera specific for the denatured glycoprotein subunits of swine class I and class II major histocompatibility complex (MHC) antigens have been prepared and utilized to monitor changes in the mobilities of these polypeptides on SDS-polyacrylamide gel electrophoresis subsequent to various deglycosylation procedures. This information, in combination with lectin reactivity patterns for the glycoproteins bound to nitrocellulose, has made it possible to define specific structural features of the MHC antigen-associated carbohydrate side chains. Both the class I heavy (alpha) chain and the class II light (beta) chain bear a single, N-linked, complex-type oligosaccharide which reacts with lentil lectin (LcH), but not concanavalin A (Con A); a reactivity pattern suggesting the possibility of a special triantennary structure. In contrast, the class II heavy (alpha) chains appear to possess two carbohydrate units, one an N-linked, LcH-reactive, complex-type side chain, and the other, an N-linked, Con A-reactive, high-mannose-type of oligosaccharide. The data suggest considerable homology between the swine and human MHC antigens with respect to the structure of their carbohydrate side chains. The analysis also serves to illustrate how antibodies specific for the denatured polypeptide backbone of individual glycoproteins, along with lectin reactivity patterns, can be used to extract structural information about the attached carbohydrate moieties using minimal amounts of partially purified glycoproteins.