AbstractHBeAg, which is a hepatitis B‐associated antigen, was purified and characterized. The end product from ion exchange, gel filtration and immunoadsorbent columns consisted mainly of immunoglobulin G (IgG) molecules, identified by immunoelectrophoresis and polyacrylamide gel electrophoresis. At least four polypeptides with molecular weights of 55,000, 38,000, 25,000 and 20,000 daltons were detected in four different preparations of HBeAg. Two of these have molecular weights similar to the heavy and light chains of IgG (55,000 and 25,000 daltons). It is postulated that HBeAg may be a small ligand of 20,000 daltons with an affinity for IgG. This affinity, especially if it involves the light chain of IgG, could account for the 38,000 or 42,000 molecular weight polypeptides seen by autoradiograph