This study was undertaken as a check on an earlier study of the relationship between coal smoke soot and tumors of the lung in mice (1). In the earlier work, we reported an 8 per cent incidence of tumors of the lung in mice exposed to soot, as contrasted with only a 2 per cent tumor incidence in control animals. We found, during the course of the earlier experiment, that the tar content of our soot varied from time to time; it was discovered, also, that our mice were more tumor-sensitive than we had thought them to be, and we furthermore realized, late in the experiment, that we knew nothing about the carcinogenic power of the particular soot we were using. In order, therefore, to rule out these uncertain factors, we used, in this our second study, a more definitely controlled genetic strain of mice, resistant to lung tumors, and we substituted for the soot a bland tar-free lampblack to which we added a known quantity of a tar known to be carcinogenic. Materials and Methods For the problem we chose 150 six-months-old mice of the Bar Harbor C57 Black strain, a strain markedly resistant to lung tumors, with a reported spontaneous lung tumor incidence of less than 1 per cent. This particular strain of mice was selected because the incidence of lung tumors had been more accurately, or rather more purposefully, studied than in the Old Buffalo strain used in our earlier experiment. One hundred of these animals were placed in cages in which sawdust and shavings bedding was replaced by a tar-free lampblack to which was added 10 per cent pure gas-works tar of known carcinogenic activity. The tar was dissolved in benzene and sprayed into the lampblack. The mixture was made up in small quantities to assure accuracy, and each batch was left uncovered until the benzene had evaporated from the tar-laden compound. Originally the amount of tar used was 15 per cent, but the mouse mortality rate was immediately so high that the quantity was reduced to 10 per cent.