cTreatment options for infections due to carbapenem-resistant Acinetobacter baumannii are extremely limited. Minocycline is a semisynthetic tetracycline derivative with activity against this pathogen. This study compared susceptibility testing methods that are used in clinical microbiology laboratories (Etest, disk diffusion, and Sensititre broth microdilution methods) for testing of minocycline, tigecycline, and doxycycline against 107 carbapenem-resistant A. baumannii clinical isolates. Susceptibility rates determined with the standard broth microdilution method using cation-adjusted Mueller-Hinton (MH) broth were 77.6% for minocycline and 29% for doxycycline, and 92.5% of isolates had tigecycline MICs of <2 g/ml. Using MH agar from BD and Oxoid, susceptibility rates determined with the Etest method were 67.3% and 52.3% for minocycline, 21.5% and 18.7% for doxycycline, and 71% and 29.9% for tigecycline, respectively. With the disk diffusion method using MH agar from BD and Oxoid, susceptibility rates were 82.2% and 72.9% for minocycline and 34.6% and 34.6% for doxycycline, respectively, and rates of MICs of <2 g/ml were 46.7% and 23.4% for tigecycline. In comparison with the standard broth microdilution results, very major rates were low (ϳ2.8%) for all three drugs across the methods, but major error rates were higher (ϳ5.6%), especially with the Etest method. For minocycline, minor error rates ranged from 14% to 37.4%. For tigecycline, minor error rates ranged from 6.5% to 69.2%. The majority of minor errors were due to susceptible results being reported as intermediate. A cinetobacter baumannii has become a major health care-associated pathogen over the past 2 decades, due to its intrinsic resistance to several classes of antimicrobial agents, its propensity to acquire resistance to other drug classes, and its ability to resist desiccation in environments typically found in hospitals (1). A. baumannii causes a variety of infections, with respiratory tract infections being the most common (1). Significant clinical challenges are posed by carbapenem-resistant A. baumannii, which accounted for over 75% of A. baumannii clinical isolates tested in a recent global survey (2). Mortality rates for carbapenem-resistant A. baumannii infections may be as high as 76% (3). Treatment options for carbapenem-resistant A. baumannii infections have not been well defined but generally include polymyxins (colistin and polymyxin B), tigecycline, or sulbactam, alone or in combination with a second agent, such as rifampin or a carbapenem, with the expectation of synergistic activities (4). However, toxicity (polymyxins), suboptimal pharmacokinetics (tigecycline), and the propensity for development of resistance (sulbactam) limit these options (1).Minocycline is a semisynthetic tetracycline derivative that was introduced into clinical practice in the 1960s (5). With the recent reintroduction of an intravenous formulation of minocycline, there is increasing interest in this agent as an additional treatment option for carbapenem-resi...