Purpose: There was little evidence that Tc-99m labeled ciprofloxacin (Infecton) located inside of bacteria. Antimicrobial activity of Infecton could be an indirect evidence of its location. We compared in vitro antimicrobial activities of Infecton and ciprofloxacin. Materials and methods: Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of Infecton and ciprofloxacin against three standard strains of bacteria, Staphylococcus aureus ATCC 29213, Escherichia coli ATCC 25922 and Pseudomonas aeruginosa ATCC 27853 were measured using modified broth macrodilution techniques and pour plate methods, respectively. Data were expressed as mean±SE (range). Results: MICs of Infecton and ciprofloxacin were 1.12±0.20 (0.8∼1.6) ㎍/㎖ and 0.36±0.04 (0.2∼0.4) ㎍/㎖ for S. aureus, 0.03±0.005 (0.025∼0.05) ㎍/㎖ and 0.011±0.001 (0.006∼0.012) ㎍/㎖ for E. coli, and 0.96±0.16 (0.8∼1.6) ㎍/㎖ and 0.56±0.098 (0.4∼0.8) ㎍/㎖ for P. aeruginosa, respectively. MBCs of Infecton and ciprofloxacin were 2.56±0.39 (1.6∼3.2) ㎍/㎖ and 0.88±0.2 (0.4∼1.6) ㎍/㎖ for S. aureus, 0.04±0.06 (0.025∼0.05) ㎍/㎖ and 0.02±0.01 (0.025∼0.05) ㎍/㎖ for E. coli, and 2.24±0.39 (1.6∼3.2) ㎍/㎖ and 1.44±0.16 (0.8∼1.6) ㎍/㎖ for P. aeruginosa, respectively. Conclusion: Although both MICs and MBCs of Infecton were higher than those of ciprofloxacin, all three standard bacterial strains were sensitive to Infecton. It could be an indirect evidence that Tc-99m Infecton be a specific imaging agent for bacterial infection. (Korean J Nucl Med 2001;35:75-80)