The various cellular components of immune rabbit histiocytes have been analyzed for their ability to induce cellular resistance in normal animals. The results of these investigations have shown that the nuclear and mitochondrial fractions were inactive and that the microsomal and ribosomal fractions were active.

The importance of ribonucleic acid in induction of cellular resistance was established by isolation of an active ribosomal RNA and by demonstration of inactivation of this material with ribonuclease but not with deoxyribonuclease or trypsin.

The possibility that viable bacilli were present in immune ribosomes was tested; the absence of complement-fixing antibodies and of skin reactivity to tuberculin in animals inoculated with ribosomes was considered as partial evidence of absence of living bacilli.

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