The external application of tar to a number of separated areas on the surface of mice, in such fashion that no single area is irritated sufficiently long to cause lesions of the skin, has resulted in a very high incidence of lung tumors. This incidence ranged from 60.0 per cent in one experiment to 78.3 per cent in another. Control mice from the same stock but from 3 to 6 months older, and for that reason the more liable to spontaneous lung tumors, failed to show a single instance of such growths. Even in a stock in which spontaneous lung tumors had been frequent, the incidence for corresponding age periods has never been above 5.5 per cent while the average has been between 1 and 2 per cent over a period of years. The tumors in the tar-painted animals occur as small white nodules, either single or multiple. They are typical epithelial neoplasms, identical histologically with those described by previous authors as occurring spontaneously in mice.
Some possible factors in the causation of the tumors are briefly discussed.