Quinupristin/dalfopristin (RP 59500, Synercid) is a parenteral streptogramin combination antimicrobial that possesses a synergistic and often bactericidal action against many Grampositive species. In this study, a collection of 1270 uncommonly isolated or tested strains were evaluated for susceptibility to quinupristin/dalfopristin using agar dilution minimum inhibitory concentration (MIC) methods described in the National Committe for Clinical Laboratory Standards. The greatest antimicrobial activity observed for quinupristin/dalfopristin was against staphylococci, streptococci, the pathogenic neisseria, Legionella spp., Lactobacillus spp., and Peptostreptococcus spp. (MIC90 range, 0.5-2 micrograms/ml). Marginal activity (MIC90s, 4 to 8 micrograms/ml) was identified for the rarer enterococci, Leuconostoc spp., Pediococcus spp., and Streptococcus bovis. Against Haemophilus parainfluenzae, Bacteroides thetaiotaomicron, Fusobacterium spp., and Prevotella spp., the streptogramin was inactive. Although no susceptible breakpoint has been approved for quinupristin/dalfopristin, three possible breakpoints (< or = 1, < or = 2, or < or = 4 micrograms/mL) were evaluated. Acceptance of the lower breakpoints (< or = 1 or < or = 2 micrograms/mL) would limit quinupristin/dalfopristin use to staphylococci, streptococci, gonococci, meningococci, and Legionella spp. These results markedly expand the understanding of the usable spectrum of quinupristin/dalfopristin.