Hepatitis B antigen (HBsAg) in human plasma was purified by affinity chromatography through a column of Sepharose 4B which had been charged with anti-HBs. Eight different elution buffers were examined for better recovery by determining the antigen titers by solid-phase radioimmunoassay (Ausria-125). Among the buffers tested, 5M NaI, 1M acetic acid (AcOH) containing 1M NaCl (pH 2.5), and 3.5 M sodium thiocyanate (NaSCN) gave good recoveries and the mast preferable result was obtained with 1M AcOH containing 1M NaCl (pH 2.5).