In a randomized crossover study, the protein binding and serum bactericidal activities (SBAs) of vancomycin and teicoplanin against Staphylococcus aureus and Streptococcus pyogenes were investigated in six healthy volunteers. Total concentrations in serum 1 h postadministration of vancomycin and teicoplanin were 25.5 +/- 2.7 and 10.8 +/- 8.9 mg/liter, respectively; mean free concentrations were 14.6 +/- 2.0 and 0.6 +/- 0.9 mg/liter, respectively. Protein binding for vancomycin was 36.9% +/- 2.87%, and that for teicoplanin was 97.4% +/- 2.6%. SBA determined in pooled human serum at 1 h against S. aureus ranged from 1:8 to 1:32 for both vancomycin and teicoplanin. Against S. pyogenes SBA at 1 h ranged from 1:16 to 1:128 for vancomycin and 1:256 to 1:2,048 for teicoplanin. In vitro kill curve studies showed that vancomycin is slowly bactericidal and that teicoplanin is bacteriostatic. Despite having less in vitro cidal activity against the study isolates and having low or unrecordable levels of free drug in serum, teicoplanin demonstrated a similar or better SBA than vancomycin. SBA was more closely related to the total drug level (r = 0.77 for S. aureus and r = 0.79 for S. pyogenes) than the free level of teicoplanin (r = 0.59 for S. aureus and r = 0.56 for S. pyogenes). The high level of protein binding of teicoplanin did not seem to impair its antibacterial activity as measured by its SBA.