Teichomycin, a new glycopeptide antibiotic with a spectrum of activity similar to that of vancomycin, was highly active against staphylococci, streptococci and Gram-positive anaerobes (Propionibacterium acnes, Clostridium perfringens and Cl. difficile). Ninety per cent of the Staphylococcus aureus and streptococcal strains, including enterococci, were inhibited by 0.4 mg/l; 90% of Staph. epidermidis strains were susceptible to 1.6 mg/l. Vancomycin was less active than teichomycin against all clinical isolates tested. Multiply resistant strains, including methicillin-resistant Staph. aureus, were all susceptible to teichomycin and vancomycin. Teichomycin was highly bactericidal for growing cells of staphylococci and Streptococcus pyogenes and moderately bactericidal for Str. faecalis. In mice, teichomycin was well absorbed upon subcutaneous administration and had a half-life of 2.5 h. It was very effective in curing experimental mouse septicemias caused by Gram-positive bacteria (ED50 values less than 1 mg/kg).