Abstract: Microbial resistance, a world health-hazard, is dramatically increasing. Evaluation of natural products to access new, safe and effective principles to rotate or substitute with the invalidated ones is one of the scientific strategies to combat drug-resistant pathogens. With this perspective, methanolic extracts of fifty plant species of 33 families which had documented uses in Iranian Traditional Medicine, were screened for antibacterial activity against five strains of each of gram negative (G-) and gram positive (G+) bacteria. Thirty samples from 28 species in 21 families had antibacterial activity at least against one of the bacterial strains. Bioactivities were evaluated by measuring Diameter of inhibition zones in Agar well diffusion assays. Among the active plants, 32.6% were active against G-, 62% against G+ and 47.3% against both G- and G+ bacteria. Dianthus coryophyllus was active against all tested G- and G+ bacteria except Micrococcus luteus. Most susceptible G- bacteria were Klebsiella pneumoniae and Bordetella bronchiseptica and least susceptible G- bacterium was Escherichia coli. In G+ bacteria, most and least susceptible were Staphylococcus aureus and M. luteus, respectively. The extracts having more traditional usages and fairly broad spectra of activity, were used to determine Minimum Inhibitory Concentrations (MIC). The least MIC, as 0.062 mg ml-1 belonged to Myrtus communis seeds against S. aureus, Bacillus cereus and B. bronchiseptica and to Terminalia chebula ripe seeds against S. aureus. The bioactive extracts were well stable at room temperature up to 18 months. Concepts including Percent Activity, Bacterial Susceptibility Index, Average Percent of Bacterial Susceptibility and Spectral Intensity Index are proposed as new approaches for interpretation of the results.