Objectives of this study were (i) to determine if the use of tetrazolium salts could enhance the sensitivity and accuracy of MIC determination by broth microdilution method for enterococci and streptococci which have specificgrowth requirements; (ii) which salt is the most appropriate; (iii) which concentration of salt must be used; (iv) whether all bacterial species need the same conditions for the tetrazolium salt assay; and (v) to verify the relation between inoculum size and the amount of formazan produced in relation to incubation time.