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SUMMARY Phage-immune variants were detected among chromogenic acid-fast mycobacteria by the action of Mycobacteriumphlei bacteriophage. One of the strains thus obtained showed not only phage immunity, but also a marked difference in colonial appear- ance, pigmentation, antigenic composition and pathogenicity for laboratory animals as compared with the original strain. INTRODUCTION During a recent survey of the phage susceptibility of chromogenic acid-fast bacteria isolated from patients with tuberculous disease, it was found that twenty- two out of ninety-eight of these strains were affected by exposure to one or more of the six phages used. In nine instances phage-immune dissociates were isolated from hazy plaques or from isolated colonies within clear plaques. Several of the phage- immune bacteria showed some morphological differences from the parent strain. In one case, the differences were so marked that this strain became the subject of the present study. METHODS Source of the strain of Mycobacterium. The chromogenic strain of acid-fast bacterium 41308 was isolated in 1956 from the sputum of a patient who, 26 months before, presented radiological evidence of widespread infiltration with bilateral cavitation. Typical Mycobacterium tuberculosis was isolated from the sputum at that time and twice again during the first 2 months of treatment. The patient received streptomycin, isonicotinic acid hydrazide and p-aminosalicylic acid for 2 years. Because the tuberculous disease appeared to be ' arrested ', chemotherapy was suspended. Two months later, pure cultures of chromogenic acid-fast bacteria were recovered from the sputum. Source of bacteriophages. The phages used were derived from Mycobacterium phlei, M. friburgensis, M. rabinowitsch, M. smegmatis and were received from Dr E. Vandra, Budapest, Hungary, and also phages D28 and D29 received from Dr S. Froman, Olive View, California, U.S.A. Characters of the chromogenic acid-fast bacterium and phage-immune variant studied: (a) cultural requirements and colonial appearance; (b) microscopic ex- amination of the bacterial elements ; (c) comparison of their antigenic composi-

Morphological and Biological Properties of a Strain of Chromogenic Acid-fast Bacteria and of its Phage-immune Variant