Hepatitis B antigen-associated particles, isolated from sera of antigen carriers, were submitted to affinity chromatography on columns of insolubilized antibodies to normal human plasma. The (7) suggest the presence of DNA in the core of the 42-nm particles. The molecular masses of the polypeptides identified in HBAg, which has a particle weight of approximately 7 X 10-18 g (8), add up to 320,000-490,000 daltons (9, 10), corresponding to 2550 to 3935 amino acids as calculated from the amino-acid composition (8). The lower and upper numbers correspond to the d and y subtypes of HBAg, respectively. Thus the total length of cistrons coding for these polypeptides should correspond to 7,650 to 11,805 nucleotides. The (500 and 0.5 mg/ml, respectively, 25,000, 30 min at 370) and diethylether + Tween 80 (500 and 0.5 mg/ml, respec-