The in vitro synergistic activity of aztreonam in combination with piperacillin, moxalactam, cefotaxime, cefoperazone, and amikacin was examined against multiply resistant isolates of the family Enterobacteriaceae accommodating a volume of 1 ml of Mueller-Hinton broth (Difco Laboratories) were utilized. Each test isolate was grown overnight on sheep blood agar; then an inoculum of 5 x 105 CFU/ml was prepared by suspending growth to a predetermined optical density. Viable cell counts were performed with each test to verify the actual inoculum density which'was later utilized for MBC determinations. The cuvettes were immediately placed in an MS-2 analysis module and incubated at 35.5°C for 16 h with constant agitation. Optical density-versus-time plots were programmatically derived by the MS-2 system for each bacterial strain and antibiotic. The MIC was defined as the lowest concentration of drug which yielded consistent growth inhibition during the test period as evidenced by review of the kinetic plots. MBCs were determined at the conclusion of the incubation period by removing two 0.1-ml samples from each cuvette well demonstrating growth inhibition and spreading the samples over the entire surface of duplicate Trypticase soy agar (BBL Microbiology Systems) plates. Resultant colonies were counted after 18 to 20 h of incubation at 35°C. The MBC was defined as the lowest concentration of antibiotic resulting in a 23-log decrease in the number of viable organisms (99.9% kill) based on the previously determined inoculum density for each test.To determine possible synergistic effects, the separate MICs for each drug tested separately were combined in MS-2 research cuvettes as follows: