Intact phagocyte function is a pre-requisite for successful defence against infection, but paradoxically, these cells may also play a major role in the pathogenesis of the infant respiratory distress syndrome. Phagocyte function is known to be deficient in pre-term infants, who are at risk of infection as a result, but these infants are also at risk of respiratory distress syndrome as a result of surfactant deficiency. Despite this, few longitudinal studies of phagocyte function have been performed in pre-term infants. We have used lucigenin-enhanced chemiluminescence to examine the respiratory burst of mixed samples containing polymorphonuclear leucocytes and monocytes of 100 pre-term infants at 48- to 72-h intervals during their admission to a neonatal care unit. Increased polymorphonuclear leucocyte chemiluminescence was associated with respiratory distress syndrome and the use of intermittent positive pressure ventilation. Multiple linear regression analysis revealed a slight, but significant depression of chemiluminescence in association with the use of gentamicin and penicillin when stronger influencing factors such as the presence of respiratory distress syndrome were taken into consideration. Measurement of phagocyte function by sensitive luminescence assays requires very little blood and may be useful in pre-term infants to follow the severity of respiratory distress syndrome. However, it is probable that other factors such as antioxidant capacity also have an important influence on the degree of tissue damage.