The effect of vitamin A (retinyl acetate) on lung tumor development in strain A mice exposed to radiation was assessed. Four groups of 75 mice were utilized. Two groups were fed a low vitamin A diet (less than 100 IU/100 g diet) and the other 2 were fed a high vitamin A diet (800 IU/100 g diet). After 2 weeks one group maintained on the high vitamin A diet and one group maintained on the low vitamin A diet were given an acute dose of 500 rad of gamma radiation to the thoracic region. Circulating levels of plasma vitamin A in all 4 groups of mice were monitored. A difference in circulating vitamin A in the mice maintained on high and low vitamin A diet became evident by 20 weeks and continued for the duration of the experiment. Mice were killed 18, 26 and 40 weeks post-irradiation, their lungs were removed and the number of surface adenomas were counted. There was a significant increase in the number of mice bearing lung tumors and the mean number of lung tumors per mouse in the irradiated group maintained on the high vitamin A diet at 40 weeks post-irradiation as compared to the irradiated group maintained on a low vitamin A diet. Under the conditions of this experiment the development of pulmonary adenomas in irradiated strain A mice appears to relate directly to circulating levels of vitamin A.