On occasion, patients receiving the anticoagulant warfarin may require treatment with antibiotics for suspected or confirmed staphylococcal infection. Nafcilllin is a narrow-spectrum penicillin frequently prescribed for the treatment of infections caused by susceptible strains of staph. aureus and staph. epidermis. Unlike most penicillins, nafcillin is primarily metabolized in the liver but is not considered an enzyme inducer, such as the case with rifampin. The literature cites one case of warfarin resistance related to nafcillin. We recently identified the same interaction in a patient with a prosthetic cardiac valve requiring up to 30 mg warfarin daily to achieve adequate anticoagulation. The exact mechanism of this interaction is not known. This nonblind trial shall consist of 12 patients requiring chronic anticoagulation with warfarin. Patients are eligible for screening contingent upon the need for treatment with nafcillin for suspected or confirmed staphylococcal infection. The objective of this study is to evaluate the pharmacokinetics and metabolism of R- and S- warfarin enantiomers in a situation that corresponds to the therapeutic use of intravenous nafcillin in patients receiving chronic warfarin.