The feasibility of detection of carcinogenic chemicals using DNA repair synthesis of cultured human fibroblasts as measured by an unscheduled 3HTdR incorporation has been explored. Of 64 chemicals tested, 29 were proximate or ultimate carcinogens, 15 were precarcinogens that required metabolic activation, 16 were non-oncogenic compounds and 4 were of unknown carcinogenicity. All directly acting carcinogens triggered a DNA repair synthesis, whereas no unscheduled 3HTdR incorporation was observed following the application of the 16 non-oncogenic compounds. As a rule, the precarcinogens (without metabolic activation) do not elicit DNA repair synthesis. However, longer exposures and higher concentrations of the precarcinogens 2-AAF, aflatoxin B1 and sterigmatocystin gave unscheduled 3HTdR uptake. The results suggest the suitability of using repair synthesis as endpoint, and cultured human cells as subjects in a prescreening programme for chemical carcinogens.