Because the symptoms and laboratory abnormalities associated with disseminated disease due to the Mycobacterium avium complex (MAC) are nonspecific, diagnosis requires recovery of the organism from blood or other normally sterile body sites. Isolation of MAC by conventional mycobacterial culture on tubed solid medium generally takes 3–4 weeks. This interval can be decreased to 5–12 days with the radiometric BACTEC TB system and to 12–19 days with Septi-Chek AFB. For diagnosis of MAC bacteremia, blood is inoculated directly into a BACTEC TB 13A blood culture vial. For quantitative cultures, blood is collected in an Isolator lysis-centrifugation tube, and the sediment of the processed sample is inoculated onto agar plates. MAC is identified by conventional biochemical tests, which take several weeks or months, or with commercial DNA probes or chromatography, each of which provides results in a few hours. No standardized reference method for antimicrobial susceptibility testing exists, and no correlation between in vitro results and clinical efficacyis clear. Isolates appear more sensitive in broth than on agar. Evaluation of susceptibility in macrophage and animal models is helpful because drugs concentrated in cells may be more efficacious against MAC—an intracellular pathogen-than would be predicted by results in cell—free systems.