Early local connective-tissue reactions around subcutaneously implanted pellets containing different polycyclic hydrocarbons were studied. A number of morphological methods were used, which included preparation of connective-tissue spreads and differential counts of cells in these spreads, vital staining of cells with neutral red, fluorescent-microscopic examination, etc. It was found that noncarcinogenic or weakly carcinogenic hydrocarbons (anthracene and 1,2-benzanthracene) did not cause any significant alterations in the course of reactions that led to encapsulation of paraffin pellets. But, in the presence of strong carcinogens (9,10-dimethyl-l ,2-benzanthracene or methylcholanthrene) these reactions underwent striking changes. Characteristic alterations induced by these substances included: 1) Long-lasting infiltration of the tissue by lymphoid cells; 2) inhibition of the process of differentiation of fibroblasts; 3) accumulation of lipides in the cytoplasm of connective-tissue cells; 4) inhibition of the capsule formation around pellets accompanied by distorted fibrogenesis. In the first weeks “early proliferation” of connective-tissue cells arose at some distance from DMBA-containing pellets. Later, most of this proliferation underwent collagenization. After 3 to 4 months multiple “presarcomatous foci” arose from the cells remaining in the collagenized areas. Possible mechanisms of development of the early changes induced by carcinogens and the role of these changes in carcinogenesis are discussed.