For 4 months laboratory white rats were administered inhalations of polydisperse dusts of the brown coal from the Nazarovsky and Beregovsky coal deposits. Revealed were mainly identical, early marked reactions of the bronchi and lung system, which, judging from the quantitative indices, could be regarded as ++pre-fibrosis, as well as some signs of irritation on the cell elements, bronchi and lung vessels. The portion of lipids and general oxyproline in the dry lung substance in the sample group amounted to 16.3-17.2%, and 9.2% in the control group. The results in combination with the immune indices changes (complement, lysozyme, lymphocyte populations), histamine in the blood and non-specific shifts in the adrenal gland state and in the heart, proved the unfavourable action of ducts with low ash content and a minimal (no less than 1.3%) content of free silica dioxide. The reasons underlying brown coal's action in question are also thoroughly discussed.