Unscheduled DNA synthesis was observed in primary rat liver cell cultures treated with members of five different classes of chemical procarcinogens requiring enzymatic activation as well as with a direct-acting carcinogen. In total, ten carcinogens and one related analog not commonly accepted as carcinogenic were active, while one weak carcinogen and four noncarcinogens were inactive. The production of unscheduled DNA synthesis by this spectrum of chemical carcinogens indicates that these cultures have substantially retained the metabolic capability of liver for activating diverse procarcinogens. Thus, such cultures may be useful for detecting the ability of chemicals to interact with DNA and, thereby, assigning them priority for consideration as potential cancer-causing agents.