The work presents the results of the study of experimental Mycoplasma pneumoniae infection in guinea-pigs. A considerable increase in the engulfment of mycoplasmas and blood leukocytes was found to occur on days 14-28 after the infection. The correlation between the degree of the engulfment of mycoplasmas by macrophages and the activity of lymphocytes in the reaction of blast-cell transformation with phytohemagglutinin and Mycoplasma antigen was observed. The peculiarities of the course of infection in sensitized animals were revealed. Virulent Mycoplasma pneumoniae strains were found to have a toxic effect on the lymphocytes and macrophages of guinea-pigs. This effect was neutralized by specific antiserum. The importance of these facts for the pathogenesis of Mycoplasma infection is discussed.