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This study was designed to evaluate the serum concentration of tobramycin sulfate following a 2.5-mg/kg intravenous infusion in 43 premature infants on days 1, 3, and 5 of age (therapy). Twenty premature infants weighing 1500 gm or less at birth and 23 others whose birthweights ranged from 1501 to 2500 gm made up the study population. Serum tobramycin levels were measured by an enzymatic immunoassay (EMIT) at one, four to six, and 12 hours after injection. Peak serum levels increased from day 1 (means, 5.2 +/- 2.2 mcg/ml) to day 3 (means, 6.1 +/- 2.6 mg/ml) and then remained unchanged at day 5 (means, 6.1 +/- 2.4 mg/ml). Approximately 40% of the study population presented trough levels above 2 mcg/ml on day 1 and over 70% on days 3 and 5. No evidence of renal toxicity or auditory dysfunction was observed. In light of the high trough levels observed during the first week of life in premature infants, it may be judicious to monitor serum tobramycin concentration and to decrease the dosage or to prolong the dose interval in order to maintain trough concentrations below 2 mcg/ml.