The study of the binding of the liver carcinogen, N-acetyl-2-aminofluorene, to the DNA of the target organ-as the probable initial step in the process of carcinogenesis-has shown that three modes of interaction occur. N-Acetyl-2-aminofluorene is covalently bound with the nitrogen to the carbon 8 of guanine (I) and with the 3-position to the free NH(2)-group of guanine (II). The third mode of interaction is formed by a covalent bond between the nitrogen of 2-aminofluorene and the carbon 8 of guanine (III). In this study the different modes of interaction were measured separately in stromal and parenchymal cells of the rat liver, after a single intraperitoneal dose. The DNA was isolated from nuclei that had been separated by 1g sedimentation. In parenchymal DNA the types of interaction I and III occur in the same amounts one day after application. In stromal cells the amount of interaction I is relatively small and interaction III predominates (ratio III:I = 5). The amount of interaction III in tetraploid hepatocytes (the largest cell population in the studied rats) per mg DNA is about two times higher than in the stromal cells. While the removal of the total amount of DNA-bound carcinogen takes place at the same rate in the two cell types, a difference in rate and efficiency of repair is observed for the different types of interaction. In tetraploid hepatocytes, interaction I is almost completely removed from the DNA 2 weeks after application, while interaction III diminishes to about 1/3 during the first week but the remaining part disappears very slowly. As shown in earlier studies, interaction II remains in the DNA at a constant level.