For testing bacterial susceptibility to the streptogramin quinupristin/dalfopristin, it seems reasonable that the test reagent should contain a similar ratio of the two compounds as in the therapeutic material, i.e. 30% quinupristin and 70% dalfopristin. The precise ratio of quinupristin to dalfopristin is not critical as long as threshold concentrations of both drugs are present. In-vitro tests were not altered by minor changes in pH, incubation in CO2 or by addition of lysed horse blood to the medium. Filter paper discs containing 15 micrograms of the quinupristin/dalfopristin reagent are preferred for agar diffusion susceptibility testing in order to optimize disc tests with enterococci. Provisional interpretive criteria for pathogens not requiring blood or increased CO2 are: susceptible, inhibition zone diameters of > or = 19 mm or MIC < or = 1.0 mg/L; intermediate, 16-18 mm or MIC 2.0 mg/L; resistant, < or = 15 mm or MIC > or = 4.0 mg/L. Those criteria will need to be reassessed as clinical experiences become available. Of particular concern is the role of quinupristin/dalfopristin in the treatment of infections due to Haemophilus influenzae or Enterococcus faecalis: the clinically relevant interpretive category for each of those two species is yet to be determined.