The bactericidal activities of teicoplanin and flucloxacillin in a 50:50 mixture of human serum and Iso-sensitest broth were compared in an in-vitro pharmacokinetic model, at serum concentrations present during surgical prophylaxis. The bactericidal activity of teicoplanin with and without serum was also compared. Six strains of Staphylococcus aureus were tested. The bactericidal rate of teicoplanin in serum was significantly lower than the rate in broth alone. However, there was no significant difference in the bactericidal rates in serum of teicoplanin compared with flucloxacillin, an antibiotic which is commonly used as prophylaxis for certain surgical procedures.