PURPOSE: The proliferative activity of cells in enzyme altered fdegrees Ci of the rat hepatoma model was measured by double immunohistdegrees Chemical staining methods using anti-bromodeoxyuridine (BrdU) and anti-glutathione S transferase of placental form (GST-P). The aim of this study was to compare the cell proliferative activity in GST-P positive altered fdegrees Ci and in negative fdegrees Ci. MATERIALS AND METHODS: Eight-week-old male Sprague-Dawley rats were administered by 200 mg/kg diethylnitrosamine (DEN) intraperitoneally, and followed by 0.02% acetylaminofluorene (AAF)-containing diet for 4 weeks. One week after administration of AAF diet, two-thirds hepa tectomy was performed. Control animals were treated as same except for the omission of AAF in the diet. The rats were sacrified 0, 1, 3, 5, 7, 14 and 21 days after partial hepatectomy. The slices of liver were fixed in acetone, dehydrated in benzene and stained by peroxidase-anti peroxidase method against GST-P and by avidine-biotin peroxidase complex method against BrdU. RESULTS: The area of the GST-P positive fdegrees Ci was increased during the experimental period. In the experimental group, the S-phase fraction in the fdegrees Ci remained high during the first week and was decreased thereafter. However, the GST-P negative area maintained a low S-phase cell frac tion throughout the experimental period. CONCLUSION: These results suggest that hepatic cells in the enzyme altered fdegrees Ci may escape a suppressor effect of AAF in contrast to the normal cells in which their growth are inhibited by AAF.