As part of the Artemis project, 11 500 isolates (3000 from patients with respiratory tract infections) were collected throughout six European countries between 1994 and 1996. Twenty-seven hospitals or laboratories participated in this first phase of the study. The activities of three classes of antimicrobial agents (fluor-oquinolones,-lactam agents, macrolides) are presented for the six most frequently isolated pathogens (Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenzae, Moraxella catarrhalis, Pseudomonas aeruginosa, Klebsiella pneumoniae). Overall, trovafloxacin and ciprofloxacin activities were similar forHaemophilus influenzae, Moraxella catarrhalis andKlebsiella pneumoniae isolates. Of theStreptococcus pneumoniae isolates, 6% were resistant to penicillin. Trovafloxacin had the highest activity against theStreptococcus pneumoniae isolates, with a minimum inhibitory concentration of 0.25 mg/l for 90% of isolates (MIC₉₀); all strains tested were susceptible to trovafloxacin. The MIC₉₀ of ciprofloxacin forStreptococcus pneumoniae was 3 mg/l, and overall 52% of the strains were susceptible; 9% were resistant. Azithromycin and clarithromycin exhibited similar activity against all collected pathogens, exceptHaemophilus influenzae. All strains ofHaemophilus influenzae were susceptible to azithromycin compared with 79% for clarithromycin, with respective MIC₉₀s of 2 and 16 mg/l. The data presented demonstrate differences in the susceptibility patterns of six major respiratory tract pathogens in Europe.