Sera of patients with acute and chronic hepatitis B and antigenemia were tested for the presence of anti-HBe and anti-HBs antibodies, free or bound in immune complexes. The possible occurrence of immune complexes of HBcAg in these sera was also investigated. Immune complexes were identified by antigen specific enzyme immunoassay, in which polyclonal antibodies against synthetic fragments of proteins S and C of HBV and mono- and polyclonal anti-HBc antibodies were used as a solid phase. Free and/or antigen bound anti-HBe antibodies were detected in 100% of patients with acute (81% HBeAg positive) and in 37% of patients with chronic hepatitis, all HBeAg positive. Anti-HBs antibodies or their immune complexes were found in 83% and 37% of patients, respectively. In not any patient circulating complexes of HBcAg could be identified. The results obtained support the observations that humoral immune response to HBeAg and HBsAg can be detected earlier than generally accepted; they also suggest that the detection of anti-HBs in a single sample of serum should not be considered as the evidence of elimination of infection.