ABT-773 is a novel ketolide effective against antibacterial-resistant respiratory tract pathogens. The pharmacokinetic profile of ABT-773 was studied in rats and consisted of a mean peak concentration in plasma of 1.07 microg/ml and an area under the concentration-time curve (AUC) of 12.03 microg. h/ml when the compound was delivered at a dose of 25 mg/kg of body weight. It concentrated in rat lung tissue, with a lung tissue-to-plasma ratio of 29 based on the AUC. In acute systemic infections in mice, ABT-773 showed efficacy against macrolide-susceptible strains of Staphylococcus aureus, Streptococcus pneumoniae, S. pyogenes, and Listeria monocytogenes. Additionally, ABT-773 improved the survival of mice infected with resistant S. pneumoniae containing either the ermB gene, the mefE gene, or altered penicillin binding protein genes. In a rat lung model of infection, ABT-773 demonstrated 50% effective doses lower than those of comparator macrolides when evaluated against the following strains of S. pneumoniae: a macrolide-lincosamide-streptogramin B-susceptible strain, an ermB strain, and an mefE strain. ABT-773 was also effective against Haemophilus influenzae lung infections in rats. Thus, ABT-773 may prove to be a useful new antibacterial agent for the treatment of respiratory tract infections.