The tumor-producing activity of local applications of dithranol, 10-propionyl dithranol and butantrone was studied in 420 female white NMRI mice. An initiation with 20 micrograms of 7,12-dimethyl-benz(alpha)anthracene (DMBA) was followed 2 weeks later by three applications per week of the test compounds in 50 microliters of acetone for 50 weeks. Several control groups receiving only acetone or DMBA and test compounds without DMBA were included. Dithranol and 10-propionyl dithranol caused transient, although serious, skin irritation during the first 2 weeks of the treatment. Hyperplasia was a common finding in the same groups at the end of the treatment. Dithranol (3.5 mM) induced 11 papillomas in 8 mice (26.7%) without DMBA and 29 papillomas in 17 mice (56.7%) with DMBA. 10-Propionyl dithranol was tumorigenic as well: 3.5 mM caused 15 papillomas in 11 mice (36.6%) without DMBA and 28 papillomas in 17 mice (56.7%) with DMBA and 1.5 mM with DMBA caused 7 papillomas in 6 mice (20%). In the butantrone groups, there was only one single papilloma with the 1.5 mM concentration plus DMBA. It is concluded that, in contrast to dithranol and 10-propionyl dithranol, butantrone (3.5 mM) is not tumorigenic in the dorsal murine skin.