The diagnostic value of an anti-mu-capture immunoassay for the detection of IgM antibody against hepatitis B core antigen (anti-HBc) was evaluated. Strongly positive results were obtained from the acute phase sera of the 25 acute hepatitis B patients who were hepatitis B surface antigen (HBsAg) positive and of the 18 confirmed acute hepatitis B patients who had already cleared HBsAg when symptoms developed. Negative results were obtained in 5 hepatitis A patients, 20 non-A, non-B acute hepatitis patients serologically susceptible to HBV, 22 patients with chronic hepatitis B liver disease, 15 asymptomatic HBsAg carriers, and 10 healthy patients immune from past HBV infection. Fourteen of the acute hepatitis patients remained HBsAg positive for a follow-up period of at least 6 months, and 12 of these were found consistently anti-HBc IgM negative. These were considered as chronic HBsAg carriers with a superimposed form of acute liver injury. These data show that this assay can differentiate between acute from chronic (HBsAg positive) and recent from old (HBsAg negative) hepatitis B virus infection. Thus, it should be very useful in the complex diagnostic situations encountered commonly in areas with high prevalence of HBV infections.