DNA polymerase alpha (DNA-P alpha) in the nuclei of hepatocytes was visualized by the immunoperoxidase method to study the number of liver cells which were at the stage of G1, S, and G2 stage in the cell cycle. Seven liver specimens from patients with acute hepatitis (AH), 17 with chronic persistent hepatitis (CPH), 32 with chronic active hepatitis (CAH), 6 with liver cirrhosis (LC), 4 with hepatocellular carcinoma (HCC) and 4 with hospital controls were studied. The number of DNA-P alpha-positive hepatocytes in 1000 hepatocytes were as follows: 19.1 +/- 18.0 in AH, 8.8 +/- 6.1 in CPH, 27.3 +/- 23.8 in CAH, 21.8 +/- 14.3 in LC, 545.3 +/- 184.0 in HCC and 1.1 +/- 1.1 in hospital controls. The number of DNA-P alpha-positive hepatocytes in HCC were significantly increased compared with other liver diseases. Likewise, those in CAH and LC were higher than those in CPH and hospital controls. The liver cell necrosis was thought to be one of the secondary stimulators for cell division of hepatocytes.