The X-irradiation of cultured Syrian hamster cells followed by exposure to a chemical carcinogen after seeding for colony formation results in an enhancement of transformation ordinarily associated with the chemical. Maximum enhancement occurred when 48 hr separated the two treatments and when 250 R was used. With different concentrations of benzo(a)pyrene (0.1-5 μg/ml medium) a similar factor of enhancement was obtained with 250 R, which averaged 8.35, implying that the irradiation pretreatment caused a constant multiplying factor. These results are consistent with the interpretation that transformation occurs through interactions in keeping with a one-hit hypothesis. The transitory effect of irradiation and the lack of transformation with irradiation alone argues against the selection of a special radiation-sensitive cell. At the time of maximum enhancement (48 hr), the cell cycles of irradiated and nonirradiated cells do not differ. The addition of benzo(a)pyrene to irradiated cells does not alter the number of cells capable of synthesizing DNA or the flow of cells from G 1 to S. The number of cells accumulated in mitosis also does not appear to differ between irradiated and nonirradiated cultures treated with 2.5 μg benzo(a)pyrene. Chromosome aberrations or alterations in ploidy cannot account for the significant increase in enhancement observed at 48 hr postirradiation; however, twice as many cells with chromosome or chromatid gaps were found at 48 hr as compared to either 24 or 72 hr postirradiation.