BACKGROUND The present study was carried out to evaluate the antimicrobial activity of a synthesized 2-methylthio-benzo[g][1,2,4]triazolo[1,5- a]quinazoline series. The compounds (1-21) were tested against a variety of Gram-positive bacterial species including Bacillus subtilis (RCMB 01001 69-3), Enterococcus faecalis (RCMB 0100154-2), Staphylococcus aureus (RCMB 0100183-9), Staphylococcus epidermidis (RCMB 0100183-9) and Streptococcus pyogenes (RCMB 0100172-5). In addition, Gram-negative bacteria were also tested including Pseudomonas aeruginosa (RCMB 0100243-5), Escherichia coli (RCMB 010052-6), Proteus mirabilis (RCMB 01002 54-2), Klebsiella oxytoca (RCMB 01002 83-4) and Enterobacter cloacae (RCMB 01002 64-5). Furthermore, their activities were screened against ten types of fungi i.e. Aspergillus fumigatus (RCMB 02568), Syncephalastrum racemosum (RCMB 05922), Geotricum candidum (RCMB 05097), Candida albicans (RCMB 05036), Aspergillus niger (RCMB 02724), Cryptococcus neoformans (RCMB 05642), Candida tropicalis (RCMBA 05239), Penicillium expansum (RCMB 01924), Microsporum canis (RCMB 0834) and Trichophyton mentagrophytes (RCMB 0925). METHODS Evaluation of antimicrobial activity was performed using agar well diffusion method in comparison with ampicillin and gentamycin as antibacterial reference drugs, and amphotericin B as antifungal reference drug. The minimum inhibitory concentration (MIC) was determined using the broth double dilution technique. RESULTS The MIC values of the tested compounds were reported in .g/mL in which ampicillin, gentamicin and amphotericin B were used as standard reference drugs. The obtained results revealed that compounds 3, 4, 7, 8, 9, 10, 11, 14, 17, 18, 19, 20 and 21 showed significant antimicrobial activity against six bacterial and six fungal strains. CONCLUSIONS The promising compounds could be employed as useful scaffolds for building of new derivatives with more potent antimicrobial effects.