Measurement of 'unscheduled' DNA synthesis in HeLa cells by liquid scintillation counting after carcinogen treatment.

HeLa cells, conditioned in an arginine-deficient medium to reduce DNA S-phase synthesis, were treated with one of four ultimate carcinogens (MNNG, BrMBA, N-acetoxy AAF and EMS) and one precarcinogen, AFB1. All treated cells preferentially incorporated [3H] thymidine as a result of DNA repair monitored by liquid scintillation counting of the extracted DNA. The cells showed some capacity to activate AFB1, but repair synthesis was much increased if a rat liver mixed function oxidase preparation was also present. At equimolar concentrations the various carcinogens stimulated different amounts of DNA repair; this variation was not proportional to the carcinogenic potency of the chemicals tested. Reasons for this are discussed as is the use of this technique as a screen for chemical carcinogens.