Thirty-seven patients were given a single, 2-g intravenous bolus injection of flucloxacillin prior to open-heart surgery. Within 12 h, flucloxacillin concentrations in serum and heart valves declined from 125.2 to 4.4 micrograms/ml and from 16.5 to 3.7 micrograms/g, respectively. Concentrations in subcutaneous tissue and muscle were almost identical, declining from 14.7 or 14.2 micrograms/g to undetectable levels after 8 to 10 h.