The in vitro antimicrobial activity of SCH 27899 (everninomycin), a novel oligosaccharide compound of the everninomycin class, was compared with vancomycin, chloramphenicol, clinafloxacin, teicoplanin and doxycycline against 428 clinical strains of bacteria. Everninomycin base exhibited the greatest antimicrobial activity compared to other formulations against all strains tested (MIC90: 0.25 microg/ml) followed by clinafloxacin and teicoplanin (MIC90: 0.5 microg/ml), vancomycin (MIC90: 2 microg/ml), and doxycycline (MIC90: 16 microg/ml). Everninomycin demonstrated the best activity against Streptococcus spp. (serogroups A, B, C, F, G) and Streptococcus pneumoniae, and lower activity against coagulase-negative staphylococci (MIC90: 0.5 microg/ml). All enterococci had an everninomycin MIC of 0.5 microg/ml or less. Everninomycin had no measurable antimicrobial activity against gram-negative aerobic organisms except Flavobacterium meningosepticum (MIC50: 2 microg/ml). Some everninomycin activity was observed against Clostridium spp., Peptostreptococcus spp., and the Prevotella bivius-disiens group. Everninomycin showed excellent activity (MIC90: 0.25 microg/ml) against the fluoroquinolone-resistant strains and all gram-positive strains resistant to vancomycin (MICs less-than-or-equal 4 microg/ml). The MBC/MIC ratios and killing curve data suggest that everninomycin is not uniformly or rapidly bactericidal. These in vitro data indicate that everninomycin could be useful against emerging gram-positive strains resistant to other contemporary antimicrobials.