Pulmonary mycobacteriosis is usually caused by Mycobacterium tuberculosis or Mycobacterium avium complex. There are, however, other slow-growing mycobacteria that can cause pulmonary infection. Mycobacterium kansasii, Mycobacterium malmoense, Mycobacterium xenopi, Mycobacterium szulgai and Mycobacterium simiae typically infect middle-aged to elderly persons with preexisting lung disease. Differentiation of infection with these five mycobacteria from infection with Mycobacterium tuberculosis, by culture and determination of the antimicrobial susceptibility pattern of the organism are important for several reasons. All five organisms are found in water and soil. They probably infect humans from environmental habitats; human-to-human spread of infection is thought not to occur. Furthermore, isolation of the organisms in culture may represent contamination of the specimen or colonization of the patient, and not necessarily an infection. Finally, although the antituberculosis drugs-isoniazid, ethambutol, rifampin and streptomycin-have been used for treatment of infection with these five organisms, there are often differences between the antimycobacterial susceptibility patterns of Mycobacterium tuberculosis and those of the non-tuberculous mycobacteria. Thus, the optimal choice of drug therapy may differ from that used for tuberculosis.