Twenty-two preterm infants of gestational age 26 to 34 weeks were treated with amikacin for suspected bacterial infection, using a dose of 7.5 mg/kg, 12-hourly, intra muscularly. Blood levels were measured using a radio-immunoassay kit capable of giving results within 4 h. One-hour peak levels showed wide variation and the mean level one hour after the first dose was 18.2 mg/1; in severe infections an initial loading dose of 10 mg/kg is therefore recommended. Trough levels in individual infants also varied considerably from day to day, but showed no overall accumulation. There was no obvious adverse effect on hepatic or renal function.