The oxazolidinone linezolid represents a new antibacterial class of potential benefit in managing multidrugresistant gram-positive infections, including those caused by Streptococcus pneumoniae. In a gerbil model of acute otitis media (AOM) induced by either penicillin-resistant S. pneumoniae (PRSP; amoxicillin MIC ‫؍‬ 8 g/ml, linezolid MIC ‫؍‬ 1 g/ml) or penicillin-susceptible S. pneumoniae (PSSP; amoxicillin MIC ‫؍‬ 0.015 g/ml, linezolid MIC ‫؍‬ 1 g/ml), we explored the plasma and ear fluid levels of linezolid required to demonstrate efficacy. Threshold pathogen doses required to induce bilateral AOM (1,500 CFU/ear with PRSP; 30 CFU/ear with PSSP) were administered to gerbils by intrabullar injection on day 0. At peak infection (ϳ10 6 to 10 7 CFU/ear flush; day 2 for PRSP-AOM and day 3 for PSSP-AOM), twice-a-day oral doses of linezolid, amoxicillin, or vehicle were administered over 4.5 days prior to collection and assay of middle ear effluents for S. pneumoniae content. Linezolid doses of >10 mg/kg of body weight induced significant cure rates of >72% versus both PRSP and PSSP infections, whereas amoxicillin at <100 mg/kg was consistently effective only versus PSSP-AOM. Plasma and ear fluid levels of linezolid necessary to elicit pneumococcal eradication from the middle ear were measured by high-performance liquid chromatography-tandem mass spectrometry and found to be similar both within and between each infection protocol. The plasma-ear fluid pharmacodynamic profile associated with linezolid efficacy was a T>MIC of >42%, a C max /MIC ratio of >3.1, and a (24-h area under the curve)/MIC ratio of >30 h. Application of this model will be useful in defining preclinical pharmacodynamic relationships of novel antibiotics necessary to cure S. pneumoniae-induced AOM.Streptococcus pneumoniae is an obligate parasite in humans that often becomes the causative organism of upper respiratory tract infections, including pneumonia, sinusitis, and acute otitis media (AOM) (2, 3). Of particular concern has been the increase in the frequency of antibiotic-resistant isolates of this ubiquitous pathogen. Since the late 1980s, a steady rise in the incidence of beta-lactam-and macrolide-azalide-resistant pneumococcal strains has been well documented in epidemiological surveillance studies (13,21,22). Currently, 40 to 50% of S. pneumoniae clinical isolates have demonstrated reduced susceptibility to penicillin while exhibiting increased cross-resistance to other antibiotic classes, such as macrolides, trimethoprim-sulfamethoxazole, and tetracyclines (13, 21). In terms of treating AOM, this resistance development has been associated with clinical failures of drug therapy (10,11,29).AOM is diagnosed over 25 million times annually in the United States, and in approximately 40% of cases the infection is caused by S. pneumoniae (4). One approach to overcoming the difficulties inherent in managing AOM induced by multidrug-resistant S. pneumoniae is the discovery of new antibiotics with improved antipneumococcal activity (23, 29). The o...