Tetracycline was administered orally in daily doses of 20 or 40 mg to 50 practically healthy male chinchilla rabbits weighing 2-2.5 kg. The thymus, appendage, spleen, ileocecal, mesenteric and axillary lymph nodes, bone marrow and peripheral blood were examined with the methods of morpho- cytometry. To verify the data the nonparametric criterion U of Wilcoxon-Mann-Whitny and the Student criterion were used. The mean curves reflecting the reaction level in the peripheral immunity system as a whole were plotted with the use of graphical analysis. It was found that tetracycline in doses of 20 and 40 mg induced a significant change in both the central and the peripheral immunity organs. Two trends in the development of the reaction in the system of immunogenesis were observed: (a) activation, transformation and differentiation of immunocompetent cells and (b) destruction of the cells, particular by transforming and proliferating cells. This gave rise to the depletion of the immunity system which was intensified if the drug was used for a prolonged period of time and in higher doses.