Chemically induced DNA fragmentation and unscheduled DNA synthesis were determined in gamma-glutamyltranspeptidase (GGT)-positive and GGT-negative hepatocytes isolated from rat livers subjected to a multistage hepatocarcinogenesis regimen (Solt-Farber), which included 0.05% phenobarbital promotion for 6 weeks (early) or 6 months (late). The results indicated that there was DNA damage in untreated GGT-positive and GGT-negative hepatocytes with either period of promotion compared with normal hepatocytes; however, no statistical difference could be seen between GGT-positive and GGT-negative hepatocytes. DNA damage induced in vitro by the activation-dependent carcinogen dimethylnitrosamine was much less in GGT-positive hepatocytes than in GGT-negative hepatocytes or normal hepatocytes. No significant difference in DNA damage was seen in both GGT-positive and GGT-negative cell populations following treatment with the activation-independent carcinogen ethylnitrosourea (ENU), although DNA damage of GGT-positive hepatocytes was less than that of normal hepatocytes. The background of unscheduled DNA synthesis in both GGT-positive and GGT-negative hepatocytes at either time of promotion was higher than that of normal hepatocytes. The capacity for DNA repair in GGT-positive hepatocytes appeared to be lower than that in GGT-negative hepatocytes. GGT-negative hepatocytes exhibited a lower capacity for DNA repair than that of normal hepatocytes in terms of the rate of unscheduled DNA synthesis elicited by dimethylnitrosamine and ethylnitrosourea in vitro.