The antimicrobial activities of colistin and other antibiotics against clinical Acinetobacter baumannii and the mutant prevention concentration (MPC) of colistin against multidrug-resistant A. baumannii were studied. All 70 stains tested were sensitive to colistin. The MPC range of colistin against 30 multidrug-resistant A. baumannii stains was approximately 32 to >128 microg/ml, and the MPC at which 90% of the isolates tested were prevented (MPC(90)) exceeded 128 microg/ml, which was much higher than the plasma concentration of colistin at the current recommended dosage. So, combination therapy for colistin treatment of A. baumannii would be prudent to slow the emergence of resistance.