Several series of chronic experiments in white mice and white rats were carried out in order to determine the carcinogenicity of Estonian oil shale soot as well as the soot from oil shale fuel oil. All the investigated samples of soot showed a relatively low (from 14 to 1200 ppm) benzo(a)pyrene content. The benzene extract of oil shale soot, painted on the skin of white mice, proved to be strongly carcinogenic: in most of the animals skin tumors developed. The benzene extract of shale fuel oil exerted a considerably weaker carcinogenic action than the extract of soot of solid shale. The effect of oil shale soots as well as of tars extracted from oil shale soot on the bronchial mucosa and lung respiratory tissue was studied in white rats. Lung tumors were induced in a considerable number of cases in a series of experiments, where the tarry material from solid fuel soot, containing 1070 ppm benzo(a)pyrene, as a suspension in an aqueous solution of Tween 40, was administered by repeated intratracheal instillation. Lung tumor incidence among various control and experimental groups of rats is presented. The peculiarities of oil shale combustion are described.