Nafcillin (150 mg/kg/day, divided every six hours) was administered intravenously to 46 patients ranging from 5 to 163 months of age with suspected or proved bacterial infections. Thirteen of 15 patients with bacteriologically proved infection responded to nafcillin. Two patients with cellulitis due to Escherichia coli or to Hemophilus influenzae type b did not improve with nafcillin therapy. A mean serum nafcillin concentration of 48 microgram/ml was observed 30 minutes after a dose of 37.5 mg/kg. The mean serum half-life was 0.76 hours. There was no significant relation between age and serum half-life, volume of distribution, or plasma clearance rate. Two patients developed neutropenia and six other patients developed an eosinophilia greater than 400/mm3 while receiving nafcillin. No significant toxic effect of nafcillin on liver or renal function was observed. One patient had phlebitis. There was a significant correlation between nfacillin concentrations and serum inhibitory and bactericidal titers (P less than 0.001). Results from this study indicate that nafcillin is a safe and effective antibiotic for the treatment of infections due to susceptible bacteria in the dosage tested.