When tested against 254 Haemophilus influenzae strains, LBM415, a peptide deformylase inhibitor, gave MIC 50 and MIC 90 values of 2.0 g/ml and 8.0 g/ml, respectively. The MICs were independent of -lactam or quinolone susceptibility and the presence or absence of macrolide efflux or ribosomal protein mutations. The MICs of LBM415 against 23 H. parainfluenzae strains were similar to those against H. influenzae. In contrast, erythromycin, azithromycin, and clarithromycin gave unimodal MIC distributions, and apart from -lactamase-negative, ampicillin-resistant strains, all strains were susceptible to the -lactams tested. Apart from selected quinolone-resistant strains, all strains were susceptible to ciprofloxacin, levofloxacin, gatifloxacin, moxifloxacin, and gemifloxacin. Resistance to trimethoprim-sulfamethoxazole was common. The potencies of all drugs against 23 H. parainfluenzae strains were similar to those against H. influenzae. Time-kill studies with 10 Haemophilus strains showed LBM415 to be bactericidal at 2؋ the MIC against 8 of 10 strains after 24 h. For comparison, the macrolides and -lactams were bactericidal against 8 to 10 strains each at 2؋ the MIC after 24 h. Quinolones were bactericidal against all 10 strains tested at 2؋ the MIC after 24 h. Against six H. influenzae strains, postantibiotic effects for LBM415 lasted between 0.8 and 2.2 h. In multistep resistance selection studies, LBM415 produced resistant clones in 7 of the 10 strains tested, with MICs ranging from 4 to 64 g/ml. No mutations in deformylase (def) and formyltransferase (fmt) genes were detected in any of the LBM415-resistant mutants.Haemophilus influenzae is an important pathogen responsible for community-acquired respiratory tract infections in children and adults, including pneumonia, acute exacerbations of chronic bronchitis, sinusitis, and otitis media (9, 23). In countries such as the United States, where the H. influenzae type b vaccine is widely used, type b has been replaced by untypeable H. influenzae strains. Haemophilus parainfluenzae strains may also play a role in the pathogenesis of acute exacerbations of chronic bronchitis (S. Sethi, personal communication).Antimicrobials used for the empirical treatment of Haemophilus infections include -lactams, macrolides, and (in adults) fluoroquinolones. The major resistance mechanism in H. influenzae in the United States and Europe is the production of -lactamase (encoded by bla TEM-1 and, rarely, bla ROB-1 ), which is present in over 41% of H. influenzae strains in the United States (17,24,27,30). Although less common, resistance to -lactams in the absence of -lactamase production (-lactamase-negative ampicillin resistance [BLNAR]) is caused by alterations (mutations) in penicillin-binding protein 3 (8, 31). Strains producing -lactamases remain susceptible to amoxicillin-clavulanate; however, clavulanate is not active against BLNAR strains. In addition, very rare H. influenzae strains producing -lactamase have been found to be resistant to amoxicillin-clavulanate (8...