U.V.-exposure of foetal hamster cells (secondary or tertiary sub-cultures) seeded for colony formation resulted in the induction of transformation. The U.V.-effect on colony-forming ability resulted in a D37 of 58 ergs/mm2. The observed transformation frequency induced by U.V.-increases approximately in proportion to increased doses in the range of 7-5-60-8 ergs/mm2 examined. When cells seeded for colony formation were treated with U.V. and later with either benzo(a)pyrene or N-acetoxy-fluorenyl-acetamide, commencing 6 min after exposure to U.V., there was neither an additive nor a synergistic enhancement of the transformation frequency ordinarily observed with either chemical agent alone. In mass cultures that were X-irradiated with 250 R, seeded for colony formation and treated with either 15 or 30 ergs/mm2 of U.V. 48 hours later, the enhancement of transformation was approximately 12- and 6-fold on a colony basis and 6- and 3-fold on a dish basis, respectively. No transformation occurred after X-irradiation only. The enhancement obtained by the pre-treatment with X-ray is imilar to that reported for the combination of X-ray and chemical carcinogens.