The antibacterial activities of DuP 105 and DuP 721, new oxazolidinone antimicrobial agents, were compared with those of beta-lactams and glycopeptides. Ninety percent of Staphylococcus aureus and Staphylococcus epidermidis isolates, including methicillin-resistant isolates, were inhibited by 4 micrograms of DuP 105 and 1 microgram of DuP 721 per ml. DuP 721 inhibited hemolytic streptococcus groups A, B, C, F, and G at a concentration of less than or equal to 1 microgram/ml, and it inhibited viridans group streptococci at a concentration of 2 micrograms/ml. Both agents inhibited Listeria monocytogenes, Corynebacterium group JK species, anaerobic cocci, and Clostridium spp. including Clostridium difficile. They did not inhibit members of the family Enterobacteriaceae or Pseudomonas aeruginosa, but the MIC for 90% of Bacteroides fragilis isolates was 8 micrograms of DuP 721 per ml.