Erythromycin is receiving renewed attention as an alternative for treatment of neonatal infections caused byUreaplasma urealyticumbecause 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 ≤1500 g and ≤15 days of age, from whomU. urealyticumwas 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