Basigin is a new member of the immunoglobulin super family with homology to both the immunoglobulin V domain and major histocompatibility complex class II antigen β-chain. Southern blot analysis indicated that the basigin gene was present as a single copy or as a few copies per mouse genome. Although a homologous gene was detected in the hamster and human, Southern and Northern blotting experiments indicated considerable species specificity in the basigin structure. The molecular weight of N-glycanase-treated basigin from embryonal carcinoma cells was about 32, 000 and was close to the value of basigin polypeptide inferred from the cDNA sequence; the result confirmed the open reading frame of basigin. Upon Western blotting, large amounts of basigin were detected in the mouse kidney as a glycoprotein bound to Ricinus communis agglutinin (RCA)-I and as a glycoprotein bound to concanavalin A; the molecular weight of the former was 38, 000-43, 000, and of the latter was 30, 000. Basigin of the molecular weight of 48, 000 was detected in RCA-I-binding glycoproteins of the liver, small intestine and spleen. Thus, different forms of basigin can be produced by different modes of glycosylation. Another source of heterogeneity of basigin may be differences in N-terminal sequences, since cDNA clones with different 5' coding sequences were identified.