The in-vitro activity of flurithromycin against common respiratory tract Gram-positive (85 strains) and Gram-negative (44 strains) pathogens, and a collection of anaerobes (125 strains) was compared with that of erythromycin, cefixime, amoxycillin, co-amoxiclav, ciprofloxacin, netilmicin and clindamycin. Flurithromycin possessed the same spectrum and potency of antimicrobial activity as erythromycin. The presence of 50% human serum in the test media enhanced the activity of flurithromycin against all isolates, with the exception of Streptococcus pyogenes. Flurithromycin induced a post-antibiotic effect (PAE) that ranged from 0.25 h (Haemophilus influenzae) to 3.5 h (Streptococcus pneumoniae) for all strains tested. The presence of serum increased or diminished the duration of the PAE, depending on the strain being analysed. No interaction between flurithromycin and the other drugs tested was observed by the checkerboard technique, but when the time-kill system was used, 35 cases of synergy were noted out of 120 tests performed (29%), of which 15 (43%) were with Moraxella catarrhalis, 12 (34%) with Staphylococcus aureus, four (11%) with H. influenzae and the remainder with S. pneumoniae and S. pyogenes. Netilmicin produced most synergic interactions. Antagonism was not detected by either methods.