The clastogenic potential of mitomycin C (MMC) was studied in rat liver cells and bone marrow cells. Male Sprague-Dawley rats were partially hepatectomized and treated with a single i.p. dose of MMC (3.5 mg/kg body weight) 7 or 24 h after the operation. Non-hepatectomized rats were also treated with the same dose of MMC 7 or 24 h after the mutagenic treatment; liver and bone marrow cells were isolated from hepatectomized rats (31 h after the operation) and bone marrow cells only from non-hepatectomized animals. The results show that, if MMC was administered 7 h before the isolation of cells, the induction was more efficient in liver cells than in bone marrow cells. At this sampling time, there was no consistent difference between the frequencies observed in bone marrow cells from hepatectomized and non-hepatectomized rats. An increase was observed in both tissues 24 h after the mutagenic treatment. At this sampling time, the effect was significantly higher in the bone marrow cells from non-hepatectomized animals than in the liver cells and bone marrow cells from hepatectomized animals. No significant difference between the two cell types from hepatectomized rats was observed. Different factors related to the cytostatic properties of MMC and/or to cell kinetics in the two cell types, probably affected by the surgical operation, may account for the differences in the yield of chromosomal damage.