Immunization of mice or guinea pigs with BCG rendered all or most of the histiocytes of these animals resistant to necrotization by virulent H37Rv; this cellular resistance was mediated by immune serum.

Immune mouse histiocytes (from BCG-immunized animals) were able to induce cellular resistance in normal homologous and heterologous (rabbit) animal species; mouse histiocytic ribosomes were also tested in the homologous species and found to be active.

Immune guinea pig histiocytes (from BCG-immunized guinea pigs) were ineffective in transferring cellular resistance to either homologous or heterologous (mouse and rabbit) animal species.

Immune rabbit histiocytes were capable of inducing cellular resistance in mice and guinea pigs; rabbit histiocytic ribosomes were also tested in normal mice and found to be active in induction of cellular resistance.

Recipient guinea pig histiocytes (from guinea pigs inoculated with immune rabbit histiocytes) were capable of inducing cellular resistance in normal guinea pigs and rabbits.

Cultivation of lysed immune histiocytes of all three animal species on glycerol-blood agar medium failed to reveal any viable BCG; this provided one additional bit of evidence against the idea that induction of cellular resistance is due to viable bacilli.

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