Acinetobacter baumannii is an increasingly nosocomial pathogen throughout the world, and the occurrence of multidrug-resistant (MDR) species is increasing. The aim of this study is to present the antimicrobial effects of a newly synthesized imidazoacridine, 11-chloro-3-methyl-3H-imidazo(4,5-a)acridine (CMIA), against MDR clinical isolates of A. baumannii. Standard dilution tubetest assay was performed to determine the MBC of CMIA for 91 clinical isolates of highly antibiotic-resistant bacteria with 28 of A. baumannii in them. The MBCs of CMIA ranged from 2.0 to 10.9 mg/l for Acinetobacter isolates while it was more than 47.9 mg/l for other clinical strains. The findings demonstrate that CMIA is a potent and selective antimicrobial agent against clinical strains of antibiotic-resistant A. baumannii.