Marijuana (Cannabis sativa I.) has become the second most widely used smoke product in the Western World1-2-3-4. Marijuana smoke, however, is inhaled in significantly lower doses than tobacco smoke and its effect is predominantly psychotomimetic with some acute toxic side effects. Nevertheless, information as to the carcinogenicity of this inhalant is needed. Since most marijuana smokers are also cigarette smokers, it needs furthermore to be determined whether marijuana smoke can potentiate the carcinogenic effect of tobacco smoke.