The clinical value of an enzyme-linked immunosorbent assay (ELISA) for the detection of anti-HBc IgM was evaluated by testing 202 sera from acute viral hepatitis B (AVHB), hepatitis B (HB), chronic hepatitis (CAH), chronic liver disease (CLD), cirrhosis, primary hepatoma, HBsAg carrier, acute viral hepatitis A (AVHA), hepatitis A (HA), non-A, non-B (NANB) hepatitis and miscellaneous conditions other than hepatic disease, and 19 additional various hepatic disease cases were examined for anti-delta. In clinical situations the accurate diagnosis of HB is not always possible and the differential diagnosis seems to be very important especially in making decisions of treatment and estimation of prognosis. In overall cases the highest positive rate of anti-HBc IgM was found in AVHB as shown as 74.3% (26/35) comparing to other conditions in which the positive rate was extremely low (2.1%). The anti-HBc IgM appeared to be highly specific to AVHB (83.9%) as compared to the other. The positive rate of HBsAg was high in AVHB, CAH and HBsAg carrier (100.0%) followed by CLD, cirrhosis and HB (up to 70.8%). The ALT activities and ALPalb fractions were significantly high in AVHB (p less than 0.005). The correlation between the positivity of anti-HBc IgM and highly abnormal ALT appeared be high. AVHB was confined mostly to 10-20 age group and the male to female ratio was about 6 to 1. Subgroup of AVHB II with positive anti-HBc IgM appeared to have a greater chance being positive for HBsAg and ALPalb. The S/N ratio of anti-HBc IgM was as high as 20 which was unique to AVHB.(ABSTRACT TRUNCATED AT 250 WORDS)