Backgrounds/Aims: HBsAg/IgG immune complexes (ICs) have been found in patients with hepatitis B virus (HBV) infection. To understand the immunologic process in patients with acute viral hepatitis (AVH) B, HRsAg/IgG ICs were quantified. METHODS: We serially quantified HBsAg/IgG ICs in patients with AVH B by ELISA method (Ag-specific), RESULTS: The ICs were detected in 6 out of 9 cases of AVH B with wide quantitative range from 24 ng/mL to 2,251 ng/mL. In the two cases of high ICs, the ICs were measured as 2,251 ng/mL in the 1st week and 540 ng/mL in the 3rd week. In 4 cases, ICs were detected in relatively low titers from 24 to 186 ng/mL. In all the 6 cases of HBsAg/IgG ICs positive, anti-HBs was all negative, and HBcAb was all positive. In 5 out of the 6 cases, HBsAg was positive. The appearance of ICs was not correlated with the level of serum SGPT or the duration of liver function recovery. CONCLUSIONS: HBsAg or anti-HBs were present as HBsAg/ IgG ICs during acute phase of AVH B, in which HBsAg or anti-HBs were not detected by conventional radioimmunoassay (RIA). The HBsAg/IgG ICs could be the viral marker complementing eonventional RIA as qualitative method. The quantitive measurement of HBsAg/IgG ICs might provide better understanding of the immunologic process of AVH B.