Yellow tetrazolium salt 3-[4,5-dimethylthiazol-2yl]-2,5-diphenyl tetrazolium bromide (MTT) is converted to a purple formazan (MTT-fromazan) showing maximum absorbance at 560-570 nm by dehydrogenase of live organisms. The author observed the MTT-formazan formation by live bacteria and tested whether the MTT assay could be used for rapid antimicrobial susceptibility tests. By testing Escherichia coli, Staphylococcus aureus, Vibrio vulnificus, and Pseudomonas aeruginosa strains, following results were obtained. 1. Live bacteria converted MTT into MTT-formazan. 2. MTT-formazan formation was different in quantity according to the genus of the bacteria but proved proportional to the imcubation time. 3. The most optimal bacterial concentration and incubation time were 5x10(5) - 5x10(7) CFU/ml and 30 min. 4. MICs by the MTT assay agreed well with those by broth dilution method when MTT was added after the bacteria were incubated with antimicrobials for 120 min. Above results indicate that MTT assay might be useful in the rapid determination of antimicrobial susceptibility for urgent life-threatening bacterial infections.