Abstract The structure of the CD38 molecule has been evaluated by one- and two-dimensional gel analysis and by enzymatic digestions. The source of the Ag was mainly membrane preparations obtained from MLC cells, from normal thymocytes, and from the plasmocytoma line LP-1. Membranes were solubilized in NP-40 and the extracts fractionated by immunoaffinity chromatography [using a specific anti-CD38 antibody (A10 mAb) covalently linked to Sepharose protein A]. The purified Ag migrated as a single chain of Mr = 45,000 not associated with beta 2-microglobulin. Two-dimensional IEF gel electrophoresis revealed five spots (isoelectric point (pI) range: 6.5 to 6.9). After neuraminidase treatment, the mobility of the five polypeptides shifted to a more basic pI. Endoglycosidase-H treatment reduced the Mr of CD38 by 20%, revealing a broader band centered at Mr = 36,000. Treatment of CD38 molecule with V8 Staphylococcus aureus protease yielded a single dominant band at Mr = 38,000 which was still reactive with A10 mAb. The CD38 molecular was trypsin-resistant in both denatured or native conditions. These results clearly show the glycoprotein nature of CD38 molecule, which includes 2 to 4 N-linked oligosaccharide chains containing sialic acid residues. Furthermore, the present data indicate that the CD38 molecule does not display an apparent biochemical polymorphism among the different CD38+ cells or lines.