[en] 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 rate of this etiological factor (total body dose and its rate). In experiments with 3000 white rats a 150 rad dose accumulated in the lungs during 15-21 months has lead to an increase in the number of cases of reticulosarcome, a 300 rad dose produces no such effects. Doses as high as 1000 to 2700 rad brings about multicentrical adenomas and glandular cancerous tumours against the background of pneumocirrhosis. But no reticulosarcomas of the lung have been noticed. Adenocarcinomas and squamous-cell carcinomas are distributed most often in the neigbourhood of accumulated radioactive dusts. The question is raised about the need of taking into account the microdistribution of energy in the lung tissue in exposure to α-active aerosols with subsequent microdosimetry; due account also should by made of detailed registration of forms of tumours and degree of their malignancy, which is not also observed by researchers in radiobiological investigations