Primary monolayer cultures of hepatocytes are very useful for both in vitro screening of cytotoxic and genotoxic chemicals and for studies on mechanisms of action of such compounds. However, culturing hepatocytes as monolayers result changes in the activity of drug metabolizing enzymes, with a reduction of cytochrome P-450 dependent enzyme activities as the most important examples of such changes. Thus, the overall metabolism of toxic chemicals in hepatocyte cultures seem to be closest to the in vivo situation in the earlier time periods after isolation. Compared to suspension cultures, monolayer cultures makes it possible to follow toxic effects of a chemical over a longer period of time. However, hepatocytes do not readily replicate in culture, making studies on gene or chromosomal mutational effects impossible. Despite these limitations, several studies have shown that monolayers of hepatocytes represent a good experimental model for studies on many aspects of the genotoxic effects of chemical carcinogens, such as the formation of covalently bound adducts to DNA, DNA breakage and DNA repair synthesis. The use of inhibitors of various drug metabolizing enzymes, have illustrated that different cellular effects of a carcinogen may be caused by different metabolites. Many aspects of modification of the carcinogenic process, such as the effects of co-carcinogens, anti-carcinogens and inducers of xenobiotic metabolism, as well as strain and species variations in metabolism, have been widely studied in hepatocyte cultures. Hepatocyte cultures have also been successfully used as a metabolic activation system in co-cultures with other cells which will respond to cytotoxic, mutagenic and/or carcinogenic metabolites. The use of monolayers of hepatocytes as metabolic activation system seems often to be more relevance to in vivo situation compared to the use of subcellular fractions in such studies. When extrapolating data from such in vitro studies to the in vivo situation it should be borne in mind, however, that cancer development may relate more to the proportion of the dose which is activated and less on the rate of activation. Furthermore, cancer development is a complex, multistage process which obviously is not only dependent on the genotoxic and cytotoxic characteristics of a chemical.