Testicles of nine-week-old male CBA/J mice were X-ray irradiated (1 Gy or 2 Gy) and were mated one week later with untreated virgin 12-week-old females of the same strain. The 1-Gy offspring (88 males and 62 females), 2-Gy offspring (100 males and 93 females) and additional offs pring (83 males and 84 females) were treated once subcutaneously with 0.1 mg/g body weight of urethane at 6 weeks of age. These three groups of off offspring showed similar incidences of lung tumors in both sexes. Depending on the doses of paternal X-ray irradiation, increasing incidences of adenocarcinoma were observed in the male 1-Gy and 2-Gy offspring groups. An increased multiplicity of lung carcinomas was observed in the male 2-Gy progeny that was statistically significant when compared with the control group. The results indicate that prezygotic testicular X-ray exposure of patemal animals causes the shift of adenoma-carcinoma sequence towards malignancy in the progeny.