Abstract: The aim of this study was to assess the in vitro activity of a number of antimicrobial agents (Sultrim, Oxytetracycline, enrofloxacin and florfenicol) produced by different Iranian manufacturers against E. coli K12. Comparison of the MICs of reference antimicrobials with Disk Diffusion (DD) results of different products made by Iranian manufacturers revealed that all products of tested antimicrobials seem to be in synergy with their reference MIC evaluation. Present data indicate that E. coli K 12 is resistant to Enrofloxacin (MIC value of 6.25 μg mL-1) by both MIC and DD evaluation.