IgM antibody to hepatitis B core antigen (IgM anti-HBc) was determined in acute and chronic hepatitis B by the immunocapture method. To avoid interference with the rheumatoid factor, F (ab')2 antibodies were used; non-specific reaction of IgG anti-HBc with anti-mu usually observed with 1/100 dilution of the serum was avoided by 1/1,000 dilution. IgM anti-HBc was positive in 100 p. 100 of the patients with acute hepatitis B (n = 32), in respectively 100 p. 100 and 70 p. 100 of the patients with complete recovery 6 and 12 months after acute illness (n = 10 and n = 10), 20.8 p. 100 of the healthy chronic HBs Ag carriers (n = 48), 80 and 85.7 p. 100 of the patients with chronic persistent and chronic active hepatitis (n = 40 and n = 14). S/N ratios (S = sample, N = negative controls) were above 5 in all patients with acute hepatitis. In patients with complete recovery, 6 to 12 months after acute illness, and in healthy chronic HBs Ag carriers, S/N ratios were above 5 in only 5 and 2 p. 100 of cases respectively whereas in patients with chronic hepatitis, the S/N values were dispersed. The main interest of the IgM anti-HBc test is to allow for the diagnosis between acute hepatitis B and acute hepatitis non B in healthy chronic HBs Ag carriers. In our patients, a S/N ratio above 5 discriminated between an acute hepatitis B and a healthy chronic HBs Ag carrier with a specificity of 98 p. 100.