One hundred and twenty-one HBsAg positive sera from cases of acute hepatitis and 76 follow-up sera were examined for anti-HBc specific IgM by a solid phase radioimmunoassay test. In addition five sera collected before the onset of hepatitis B, and series of sera from four cases of hepatitis B were studied. The results indicated that anti-HBc IgM appeared in the fortnight before the onset of jaundice, reached a maximum during the first month after onset, and usually remained, in diminishing amounts, for four months. The method described for anti-HBc IgM is a useful test in the acute and convalescent phases of hepatitis. It will generally distinguish acute infection with HBV from acute hepatitis due to another cause in an HBsAg carrier, and will also detect evidence of recent infection in patients who have lost HBsAg but have not yet produced anti-HBs. It is therefore a more specific test for acute hepatitis B than are tests for HBsAg alone.