The concentration of HBsAg in serum was determined in arbitrary units by quantitative immunoelectrophoresis and in units of optical density by UV-photometry of purified antigen. The HBsAg was purified from serum by gelchromatography and subsequently by isopycnic centrifugation in cesium chloride. The ratio of the immunoelectrophoretically measured concentration to the units of optical density was found to be constant in six different samples with both subtypes - ad and ay - and varying concentration. The concentration of protein in three purified HBsAg samples was determined after acid hydrolysis by automatic aminoacid analysis. The specific extinction was E1 280 mg/ml = 4,5 +/- 0,3. Thus the arbitrary units of immunoelectrophoresis were converted to concentration of HBsAg specific protein, expressed as mug/ml. In 540 Ausria positive serum samples, mainly from the beginning of an acute hepatitis, 10-40 mug/ml was the most frequent range of concentration. About 4% of the sera contained more than 100 mug/ml, about 20% contained between 0,005 and 0,5 mug/ml and were positive only in RIA. 10 mug/ml correlated with titers in complement fixation of 1:32/64, in counterimmunoelectrophoresis of 1:8 and immuno-diffusion of 1:2. The use of standards with a defined concentration of HBsAg would allow a better control of the sensitivity in qualitative tests and of reproducibility in quantitation.