RBx 1000075 and RBx 1000276, the new investigational oxazolidinones, have an extended spectrum of in vitro activity against Gram-positive pathogens and showed minimum inhibitory concentrations (MICs) lower than comparator drugs. MIC for 90% of the organisms (MIC(90)) values of RBx 1000075, RBx 1000276 and linezolid against the isolates tested were, respectively: methicillin-sensitive Staphylococcus aureus, 0.25, 1 and 4 microg/mL; methicillin-resistant S. aureus (MRSA), 0.5, 2 and 4 microg/mL; methicillin-sensitive Staphylococcus epidermidis, 0.25, 1 and 2 microg/mL; methicillin-resistant S. epidermidis, 0.5, 1 and 2 microg/mL; and enterococci, 0.25, 1 and 4 microg/mL. Against respiratory pathogens, MIC(90) values were: Streptococcus pneumoniae, 0.125, 0.5 and 2 microg/mL; Streptococcus pyogenes, 1, 0.5 and 2 microg/mL; and Moraxella catarrhalis, 0.5, 2 and 4 microg/mL. In vivo efficacies of RBx 1000075 and RBx 1000276 were evaluated in murine systemic infection against S. aureus (MRSA 562) and in a pulmonary infection model against S. pneumoniae ATCC 6303. In murine systemic infection, RBx 1000075 and RBx 1000276 showed efficacy against MRSA 562, exhibiting a 50% effective dose (ED(50)) of 6.25 and 9.92 mg/kg body weight for once-daily administration and 4.96 and 5.56 mg/kg body weight for twice-daily administration, respectively, whereas for linezolid the corresponding ED(50) values were 9.9 and 5.56 mg/kg body weight. In pulmonary infection with S. pneumoniae ATCC 6303, 50% survival was observed with RBx 1000075 at 50mg/kg once daily, whereas 60% survival was observed with RBx 1000276 at 50mg/kg thrice daily. The absolute oral bioavailabilities of RBx 1000075 and RBx 1000276 were 48% and 73%, with half-lives of 13.5 and 3.2h, respectively. RBx 1000075 and RBx 1000276 are promising investigational oxazolidinones against Gram-positive pathogens.