Erythromycin is receiving renewed attention as an alternative for treatment of neonatal infections caused by Ureaplasma urealyticum because of recently proved abilities of this organism to produce systemic disease in this population. Although erythromycin has been used clinically for almost 40 years, very little is known about its activity in the preterm neonate. Fourteen neonates, birth weights < or = 1500 g and < or = 15 days of age, from whom U. urealyticum was isolated from the lower respiratory tract were randomized to receive erythromycin lactobionate either 25 or 40 mg/kg/day in four divided doses at 6-hour intervals scheduled for a total of 10 days. Blood samples collected at multiple time points after initial and steady state doses were assayed for erythromycin by liquid chromatography. Minimal inhibitory concentrations (MICs) of erythromycin for the U. urealyticum isolates were determined. MICs ranged from 0.031 to 2 micrograms/ml; MIC90 = 2 micrograms/ml. Serum erythromycin concentrations met or exceeded most MICs, with peak values of 3.05 to 3.69 and 1.92 to 2.9 micrograms/ml for the 40- and 25-mg/kg/day dosage groups, respectively. Pharmacokinetic parameters were calculated after the initial dose and at steady state for both dosage groups and compared. No adverse effects thought to be related to administration of erythromycin were observed. These preliminary findings showed that erythromycin is well-tolerated, has favorable pharmacokinetic activity in the preterm neonate and should be further investigated for treatment of ureaplasmal infections.