AbstractINTRODUCTION AND BACKGROUND Our understanding of carcinogenesis in liver and other tissues has progressed steadily with the use of a sequential analytical approach (1-5). Persistent nodule hepatocytes which are potential precursors for cancer were shown to be resistant in vivo relative to the surrounding hepatocytes to cytotoxic and mitoinhibitory effects of hepatocarcinogens and hepatotoxins (6,7). A working hypothesis was that one population of initiated hepatocytes has this resistance property (1,3-9). To test this hypothesis of resistant hepatocytes as products of initiation with Farber and co-workers (1,9-13), we used single exposures to a variety of chemical carcinogens as initiators. This work showed that focal populations of hepatocytes were present in the liver after initiation which were resistant to cytotoxic and mitoinhibitory effects of 2-acetylaminofiuorene (2-AAF) and were manifest as nodular proliferations in response to resistance-selection with 2-AAF (9-14) or lasiocarpine (15). The resistant hepatocyte (RH) model became a useful bioassay for initiation (1,4,5). From these studies, a number of important conclusions about initiation could be reached: (a) cell proliferation of hepatocytes (within 3 days of initiating chemical exposure) is essential for initiation to occur (10-14), (b) the original clone of resistant hepatocytes after initiation is projected to be at most only a few cells in size (1,16), (c) these resistant hepatocyte nodules generated by initiation and resistance-selection are a rare event (1) or about 1 RH clone per 105or 106total hepatocytes exposed to an initiating carcinogen in vivo (1,4,5). In spite of the fact that resistant hepatocytes in persistent nodules arising after both initiation and resistant-selection could be readily studied biochemically, functionally (biologically), or histopathologically 07-31), initiated (resistant) hepatocytes induced by initiation alone without selection were not yet analyzable (1,4,5).