Method Group I (DMBA plus UVL) consisted of 31 mice, 14 to 16 weeks old (15 male and 16 female). Two drops (0.1 ml.) of the 0.5 per cent solution of DMBA were applied to the posterior half of the backs of each mouse. Fourteen days later, bi-weekly ultraviolet light exposures of the DMBA treated areas were initiated and continued for 67 weeks (a total of 13.33 X 10 ergs per square centimeter of mid-ultraviolet light energy). Group II (DMBA without UVL) consisted of 41 mice, 14 to 18 weeks old (19 male and 22 female), which were treated with DMBA as in Group I, but they received no ultraviolet light. Group III (acetone plus UVL) consisted of 47 mice, 11 to 14 weeks old (31 male and 16 female). Acetone, 0.1 ml., was applied to the posterior half of the back of each mouse. The ultraviolet light exposures were initiated fourteen days later and completed as in Group I (13.33 X 10 ergs per square centimeter of mid-ultraviolet light energy in67 weeks).