The mechanism of imipenem resistance of two Acinetobacter baumannii isolates (A-1 and A-24) was characterized in this study. A spontaneous revertant (A-1 (rev)) derived from isolate A-1 showed susceptibility to imipenem. beta-Lactamase hydrolysis studies showed no evidence of an imipenem hydrolyzing enzyme among A-1, A-24, A-1 (rev), or two other imipenem susceptible A. baumannii isolates. Outer membrane protein (OMP) analysis indicated decreased expression of a 33-36 kDa protein by isolates A-1 and A-24 when compared with A-1 (rev) and the other A. baumannii isolates. In conclusion, decreased expression of a 33-36 kDa OMP is associated with imipenem resistance among A. baumannii.