Ten neonates with persistent staphylococcal bacteremia (positive blood cultures for > or = 5 days despite appropriate antibiotic therapy) received intravenous (i.v.) rifampin in combination with vancomycin with or without aminoglycoside. Their mean birth weight and length of gestation were 900 g and 27 weeks, respectively. Their ages at the time of infection ranged from 6 to 64 days (mean, 26 days). The staphylococcal isolates were methicillin-resistant Staphylococcus aureus (five isolates), methicillin-susceptible S. aureus (two isolates), and coagulase-negative staphylococci (three isolates). The mean number of bacteremia days prior to administration of i.v. rifampin was 8.3 (range, 5 to 15 days), despite a mean peak vancomycin concentration of 33 micrograms/ml. The dosing of rifampin varied from 2.5 to 10 mg/kg of body weight every 12 h. The mean duration of the rifampin course was 9.7 days (range, 3 to 16 days). Of the 10 neonates, 8 (80%) had sterile blood cultures within 24 h, 1 (10%) had a sterile blood culture within 48 h, and 1 (10%) had a sterile blood culture within 5 days of being placed on i.v. rifampin. No adverse effects were noted in this small group of infants. Seven of the 10 neonates survived; three died from unrelated complications. The MIC ranges of amikacin, vancomycin, and rifampin for the isolates were 2.0 to 16, 0.5 to 2.0, and 0.0013 to 0.04 micrograms/ml, respectively. We also studied eight infants, with a mean age of 23 days, who were receiving i.v. or oral rifampin at a dose of 10 mg/kg/day. For i.v. administration, the peak serum concentration of rifampin (mean +/- standard deviation) was 4.02 +/- 1.22 microgram/ml. The mean trough level at 12 h postifution was 1.11 +/- 0.48 micrograms/ml. For oral administration, the concentrations of rifampin in serum ranged from 0.59 to 2.86 micrograms/ml (mean, 1.86 +/- 0.96 microgram/ml) at 2 h postingestion, increasing to a peak concentration of 2.8 micrograms/ml at 8 h postingestion. The mean 12-h postingestion level was 0.77 +/- 0.03 microgram/ml. From the study of this limited series of neonates, rifampin appears to be a safe and effective addition to therapy when staphylococcal bacteremia is persistent despite vancomycin treatment.