Article Subject: Antibiotic Resistance 10.30699/ijmm.13.3.194 Background and Aims: Flow cytometry is a rapid method that can analyze thousands of cells per second and can be used for determination of microbial populations and determination of bacterial antimicrobial susceptibility. In this study antibiotic resistance pattern of Acinetobacter baumannii isolates by flow cytometer was evaluated. Materials and Methods: 55 isolates of A. baumannii were isolated from clinical specimen of patients and were identified by biochemical tests. Antibiotic resistance patterns were studied by disc diffusion method and MDR strains were selected. MIC of Meropenem-Tazobactam and Piperacillin were determined. Also antibiotic resistance pattern of isolates was determined by coloring with Rhodamine-123 and flow cytometry. Excel software and STDV test were used to calculate the standard deviation of antibiotic resistance and MIC results.To evaluate the agreement between the results of antibiotic resistance pattern using disk diffusion methods and the MIC and Flow cytometry category agreement was used. Results: 98% of isolates were MDR. The MIC ranges for maropenem were 8-256 μg/mL and for Piperacilllin-tazobactam were 128-1024 μg/mL. By flow cytometry it was demonstrated that at concentrations of 8, 4 and 2 μg/mL of meropenem, only 1.96%, 1.44% and 0.59%, of cells were killed respectively. At concentrations of 64,128 and 16 μg/mL of piperacillin, 13.8%, 11.3% and 5.9%of cells were killed respectively. Reducing the number of living bacteria was observed with increasing concentrations of both antibiotics. Conclusion: The similarity between the results of flow cytometry and both agar and broth antibacterial susceptibility methods showed flow cytometry as a reliable and rapid test that can be used for this purpose.