When monocytes derived from normal guinea pigs or rabbits were infected with tubercle bacilli and cultivated in vitro, the bacilli multiplied abundantly within the cytoplasm of these cells. By contrast, intracellular multiplication of the bacilli was retarded or completely inhibited within the monocytes of rabbits or guinea pigs vaccinated with BCG. This inhibition of growth was observed with both virulent or attenuated strains of tubercle bacilli. Under the conditions used in the present study, the ability of monocytes to inhibit bacillary proliferation was the same whether serum from a normal animal or from vaccinated animals was used in the tissue culture medium. Moreover, the serum of vaccinated animals did not inhibit multiplication of tubercle bacilli within monocytes derived from a normal animal. The ability of guinea pig monocytes to interfere with intracellular bacillary proliferation was first perceptible 8 days after vaccination.