Immune mechanisms in hepatitis B virus (HBV) infection were investigated in 16 persons with and without hepatitis using tests for HBSAg, anti-HBs, anti-HBcand,125l-labeled HBSAg binding lymphocytes (ABL) in peripheral blood. Anti-HBC, which is an evidence of a recent or current HBV infection, was detected in all HBSAg positive sera. High counts of ABL correlated with the presence of anti-HB5in serum but not with anti-HBcor with HBSAg. In patients with chronic hepatitis, and in asymptomatic carriers of HBSAg, there was a trend towards low counts of ABL, which may represent partial tolerance to HBSAg in carriers of this particle. Further work on ABL for HBSAg and HBCAg should enhance our understanding of immunologic responses to the antigens of the hepatitis B virus.