IgM antibody to hepatitis B virus core antigen (IgM anti-HBc) develops during acute hepatitis B but frequently persists in chronic infections. To characterize persistent IgM anti-HBc better, 7-8S and 19S immunoglobulin fractions were prepared by rate-zonal centrifugation of sera from 17 patients with persistent hepatitis B (chronic active hepatitis) and were tested for IgM anti-HBc by a specific radioimmunoassay. In 16 sera peak activity was found in 7-8S fractions, although in 11 sera a minor peak was also present in 19S fractions. The low molecular weight of the predominant IgM anti-HBc was confirmed by gel filtration. In competition experiments, the binding of 7-8S antibody to an anti-IgM-coated solid phase was blocked more effectively by purified IgM than by purified IgG. These findings indicate that hepatitis B carriers with chronic active hepatitis have predominantly 7-8S IgM anti-HBc and represent a novel demonstration of naturally occurring 7-8S IgM with defined antiviral specificity.