The in vitro activity of MDL 62,879, a new peptide antibiotic that inhibits protein synthesis through an interaction with elongation factor Tu, against a wide range of recent clinical isolates of common aerobic gram-positive and anaerobic organisms was determined. MDL 62,879 was highly active against staphylococci (MIC for 90% of isolates [MIC90], 0.125 microgram/ml), streptococci (MIC90, 1 microgram/ml), and enterococci (MIC90, 0.03 microgram/ml). All isolates of peptostreptococci and Mobiluncus spp. were susceptible, as were most isolates of clostridia. MDL 62,879 was not active against isolates of fusobacteria or Bacteroides spp., but some isolates of Prevotella spp. and Porphyromonas asaccharolytica were susceptible.