Hepatitis B Antigen from plasma of a healthy chronic carrier was separated by gel chromatography using 6% agarose gel into fractions with different size (Fig. 1). Single fractions were further purified by isopycnic centrifugation in CsCl (Fig. 2) and characterized by electron microscopy (Fig. 3). The amount of the different particle groups was determined comparing the extinction peaks in the centrifugates (Table 1). 88% of antigen protein was located in the 18 to 25 nm spheres (Fig. 3a), 10% in tubules with a length between 30 and 100 nm and about 2% form tubules 100-200 nm long (Fig. 3b and 3c). Together with the tubules were found 40 nm spheres representing about 0,1% of the HBAg-substance (Dane paticles, Fig. 3d). The proportion of the larger antigen particle seems to be absent or very low in acute Hepatitis B patients, but is frequently found in chronic carriers.