Vancomycin and teicoplanin are two commercially available glycopeptide antibiotics. They have identical spectra of activity and similar mechanisms of action, and both are complex molecules. In vitro, vancomycin is more active against coagulase-negative staphylococci, while teicoplanin is more active against enterococci and pneumococci. The activity of the two antibiotics against Staphylococcus aureus is similar. The serum pharmacokinetics of vancomycin and teicoplanin are highly different, teicoplanin having a longer elimination half-life (40 hours versus 6-8 hours for vancomycin), but a higher degree of protein binding (90% versus 55%). We compared the two antibiotics on the basis of their inhibitory quotient kinetics, using the MIC90 values for the above bacterial species as the microbiological parameters, and the total and free (non-protein bound) serum concentrations as the pharmacokinetic parameters. The inhibitory quotient kinetics of vancomycin were always more favorable in terms of the free concentrations, even against those bacteria for which the teicoplanin MIC was lower.