IgM anti-HBc was measured by radioimmunoassay in serially collected serum samples during 20 acute exacerbations which developed in 14 patients with anti-HBe positive chronic type B hepatitis. IgM anti-HBc became positive in 12 of the 14 (86%) patients and in 18 of the 20 (90%) exacerbations, and elevation of IgM anti-HBc which was believed to be significant was also observed in the remaining two patients. In most of the patients who also had determinations of serum hepatitis B virus DNA, the hepatitis B virus DNA became positive. These results suggest that ALT elevation in anti-HBe positive chronic type B hepatitis is associated with active replication of hepatitis B virus. It is considered to be useful to measure IgM anti-HBc for the purpose of identifying the causes of ALT elevation in anti-HBe positive chronic type B hepatitis.