A batch of sera obtained from subjects with acute hepatitis B virus (HBV) infection, chronic carriers of hepatitis B surface antigen (HBsAg) who were either asymptomatic or who had chronic active hepatitis, and 32 sera from patients with HBsAg negative chronic active hepatitis were examined for the presence of antibodies against hepatitis B core antigen (anti-HBc) by radioimmunoassay (RIA). Sera containing anti-HBc were fractionated on sucrose density gradients to separate immunoglobulin M (IgM) and the titre of anti-HBc IgM was determined. In patients with acute HBV infection, anti-HBc IgM was detected during the acute phase of the illness with titres ranging from 1:128 to 1:4,096 (geometric mean titre 1:709). The titre of anti-HBc IgM fell rapidly over the following months and in most patients persisted at low levels for several years. Anti-HBc IgM was also detected in subjects with chronic HBV infection but with significantly lower titres. In asymptomatic carriers, anti-HBc IgM titres ranged from 1:4 to 1:32 (geometric mean titre 1:12), whilst carriers with chronic active hepatitis had titres ranging from 1:4 to 1:128 (geometric mean titre 1:35). By using a standardized assay procedure, the titre of anti-HBc IgM in a patient's serum may be of value in differentiating between acute and chronic HBV infection.