Thorium (Th) is one of the widely spread radioactive substances in nature, and together with other aerosols it can penetrate into the organism. The article considers relationships between various histological forms of neoplasms induced by a radioactive dust agent containing Th and the power of this etiological factor (total body dose and its power). In experiments with 3000 white rats a 150 rad dose accumulated in the lungs during 15--21 months led to an increase in the number of cases of reticulosarcoma, a 300 rad dose produced no such effects. Doses as big as 1000--2700 rad brought about multicentrical adenomas and glandular cancerous tumours against the background of pneumocirrhosis. But no reticulosarcomas of the lung were noted. Adenocarcinomas and squamous-cell carcinomas were distributed most often in the neighbourhood of accumulated radioactive dusts. The question is raised about the need for taking into account the microdistribution of energy in the lung tissue in exposure to alpha-active aerosols with subsequent microdosimetry; due account also should be made of detailed registration of forms of tumours and degree of their malignancy, which is not also observed by researchrs in radiobiological investigations.