Anti-HBc IgM was determined by a modified radioimmunoassay (RIA) in 35 patients with acute hepatitis B, 35 patients with chronic hepatitis B (7 with chronic persistent, and 28 with chronic active hepatitis), 157 HBsAg positive blood donors, and in 143 HBsAg negative but anti-HBc positive donors. The results of the RIA test were compared with those obtained by an ELISA technique. In chronic hepatitis, anti-HBc IgM was correlated with the occurrence of HBeAg, anti-HBe, Dane particles in the serum, HBsAg, and HBcAg in liver tissue and with biochemical and histological degrees of hepatic inflammatory activity. In acute self-limited hepatitis B, anti-HBc IgM (RIA and ELISA) was initially positive in all 35 patients. Twelve months after the acute illness, 94% of the patients were negative for anti-HBc IgM in the RIA test with only one patient showing a persistence of up to 18 months, whereas in the ELISA test anti-HBc IgM persisted in 17% of the patients over 2 years. In chronic hepatitis, the occurrence of anti-HBc IgM (RIA) showed a strong relation with the inflammatory activity, the anti-HBc IgM positive patients revealing a significantly more severe liver disease than did anti-HBc IgM negative patients. Anti-HBc IgM (RIA), however, did not correlate with the occurrence of HBeAg and Dane particles in the serum and HBcAg in liver tissue of patients with chronic hepatitis. Of the 157 HBsAg positive blood donors, anti-HBc IgM (RIA) could be demonstrated in 10 (6%), but in none of the 143 HBsAg negative, but anti-HBc positive donors, as compared to 43 (27%) and 9 (6%), respectively, in the ELISA test. Comparing the two test methods, the RIA exhibits higher specificity than did the ELISA due to a better blocking of nonspecific reactions, but possibly somewhat lower sensitivity. In this form, however, the RIA test is a more useful tool in the diagnosis of the different forms of hepatitis B virus infection and in determining the severity of chronic hepatitis B.