Guinea pigs were injected with spheroplasts of Mycobacterium tuberculosis and were sacrificed after various intervals to study the state of the organisms and to evaluate the extent of disease. The spheroplasts persisted in the body of the animals for a considerable length of time. Their presence, however, was not associated with either the development of tuberculin hypersensitivity or pathologic changes. It was found that the spheroplasts were pathogenic only when reverted in the animals to bacterial forms. The reversion of the spheroplasts to bacillary state and the subsequent initiation of infection were dependent on the duration of their stay within the host.