The experimental study made on 185 guinea pigs revealed that during the chemotherapy of destructive tuberculosis the typical bacterial forms of M. tuberculosis rapidly disappeared, but the cavities of destruction were retained and the presence of the filter-passing forms of M. tuberculosis, along with antigenemia and specific cell-mediated and humoral immunity could be detected. The use of repeated biological subculturing resulted in the reversion of the filter-passing forms of M. tuberculosis to their typical bacterial forms; simultaneously the increase of antigenemia (from subculture to subculture) and the enhancement of immune responsiveness occurred.