The 19S and 7-8S forms of IgM antibody to hepatitis B core antigen (IgM anti-HBc) were separated by rate-zonal centrifugation from the serum of 20 Greek hepatitis B surface antigen (HBsAg) carriers with a superimposed acute icteric hepatitis positive for IgM anti-HBc by a radioimmunoassay. Serological markers of hepatitis A virus (HAV), hepatitis B virus (HBV) and hepatitis D virus (HDV) infections were detected with radioimmunoassays and serum HBV DNA was detected with molecular hybridization techniques. Eighteen of the 20 carriers showed a predominance of one or the other form of IgM anti-HBc. Low molecular weight (7-8S) IgM anti-HBc was observed more frequently in HDV superinfection (5/9) and was related to a low mortality (1/9). In contrast, 19S IgM anti-HBc was observed more frequently in reactivation of chronic hepatitis B (6/9) and was related to a high mortality (5/9). These preliminary data show that in HBsAg carriers with a superimposed acute icteric hepatitis, predominance of 19S IgM anti-HBc is frequently associated with a severe clinical course; the opposite is true for predominance of 7-8S IgM anti-HBc.