Quinupristin/dalfopristin is a new water-soluble streptogramin antimicrobial agent comprising quinupristin and dalfopristin in a ratio of 30:70. The in-vitro spectrum of activity includes most multi-resistant Gram-positive aerobes, important Gram-negative aerobes, Gram-positive anaerobes and intracellular bacteria that are causal agents of respiratory, blood and cutaneous infections. Of particular note, quinupristin/dalfopristin is active against multidrug-resistant isolates of Staphylococcus aureus, Staphylococcus epidermidis and Enterococcus faecium, and against penicillin-resistant and/or erythromycin-resistant Streptococcus pneumoniae. The combination is also active against staphylococci showing both constitutive and inducible erythromycin resistance. Bactericidal activity and a prolonged post-antibiotic effect have also been noted for quinupristin/dalfopristin against Gram-positive cocci. Gram-negative bacteria susceptible to quinupristin/dalfopristin include Moraxella catarrhalis, Legionella spp. and Mycoplasma spp. Overall, the spectrum of antibacterial activity indicates a potential role for this combination in the treatment of difficult-to-treat Gram-positive infections, including those caused by multidrug-resistant organisms. Since this activity extends to Gram-negative respiratory bacteria, quinupristin/dalfopristin may also find a role in the treatment of atypical, as well as typical, pneumonia.