Evidence in the literature from in vitro and in vivo studies as to whether or not radiofrequency radiation (RFR) in the microwave range is mutagenic is predominantly negative, with some positive reports. No evidence is available as to whether RFR will alter the mutagenic activity of genotoxic chemicals during a simultaneous exposure, a likely real-life situation. Two hypotheses have been proposed: a) that RFR by itself can cause mutations in a mammalian cell in vitro assay system; and b) that a simultaneous exposure to RFR during a chemical treatment of the cells with a known genotoxic agent, mitomycin C (MMC), will alter the extent of mutagenesis induced by the treatment of the cells by the chemical alone. These studies were performed using the forward mutation assay at the thymidine kinase locus in L5178Y mouse leukemic cells. The pulsed wave RFR was broadcast from an antenna horn at a frequency of 2.45 GHz. The power density was 48.8 mW/cm2 and the measured specific absorption rate (SAR) in this system was 30 W/kg (600 W forward power), which is well above current safety guidelines. The conclusions from five different experiments, employing three different concentrations of MMC, were that a) RFR exposure alone, at moderate power levels which resulted in a temperature increase in the cell culture medium of less than 3 degrees C, is not mutagenic; and b) when cells are simultaneously treated with MMC and RFR at these same moderate power levels, the RFR does not affect either the inhibition of cell growth or the extent of mutagenesis resulting from the treatment with the chemical MMC alone.