Vancomycin hydrochloride was infused intravenously (i.v.) over a 30-min period in five horses at doses of 6.6, 11.0 and 15.4 mg/kg. Vancomycin concentration in plasma and synovial fluid samples was measured using a polarization immunoassay. A pharmacokinetic model was developed to accommodate the special features of the present study. The data were described by a two compartment open model with synovial fluid as an additional compartment in exchange with plasma. Minimum inhibitory concentration (MIC) and minimum bacterial concentration (MBC) were measured for Staphylococcus aureus and Enterococcus sp. using isolates from hospital patients. Based on the pharmacokinetic model and MIC/MBC data, a practical therapeutic protocol for vancomycin administration was established at doses of 4.3-7.5 mg/kg given as a 1-h infusion every 8 h.