The effects of tar produced from commercial Colombian black tobacco cigarettes were compared with those of tar produced from commercial U.S. blended tobacco cigarettes. The tobacco tars were fractioned to produce “refined tar” and a residue. Only one tumor was seen in 100 mice painted with an 8% solution of one or the other of the two residues. In contrast, 176 of 400 mice developed tumors when painted with either 8% or 4% of the refined tars in acetone. Both solutions of refined Colombian tar provided a tumor incidence of 62% and a carcinoma incidence of 21%. Of refined U.S. tar the 4% solutions provided a tumor incidence of only 27% and a carcinoma incidence of only 6% and the 8% solution provided a tumor incidence of 55% and a carcinoma incidence of 20%. The latent period for tumors produced by the refined Colombian tar was somewhat shorter than that for tumors produced by the refined U.S. tar. Chemical analyses of the tar did not disclose the basis for the higher carcinogenic activity of the Colombian tobacco tar. The differences in mortality between Cali and the United States and Europe are apparently due to differences in the smoking pattern of the two populations.