Changes in plasma fibronectin concentrations were determined during bacterial septicaemia in extremely preterm infants. The study was a prospective study of fibronectin concentrations in infants of less than 30 weeks' gestation. Concentrations were determined at birth, before sepsis, and throughout the episode of sepsis. Fibronectin concentrations at birth or immediately before sepsis were not significantly different between those infants who developed septicaemia and those who did not (98 (15) v 97 (10) ,μg/ml). In the infants with septicaemia, fibronectin concentrations decreased significantly on day 1 (106 (13) v 173 (18) ,μg/ml for the controls) and remained significantly lower on day 2 (123 (26) v 201 (17) ,μg/ml). By day 5 fibronectin concentrations had increased and were no longer statistically different from controls. Fibronectin is a key modulator of the immune response, with important functions in neutrophil adhesion, bacterial opsonisation, T cell activation, and vascular integrity. Acute depletion of plasma fibronectin during sepsis in preterm neonates may further abrogate their ability to control sepsis.