After birth, host defences must be recruited to manage the transition from an almost sterile to a normal environment. The present study was undertaken to evaluate the relationship between cytokine plasma levels and phagocyte burst in mothers and neonates during the peripartal period. Plasma levels of interleukin (IL)-1, IL-6, tumour necrosis factor (TNF)-alpha, interferon (IFN)-gamma, and granulocyte-macrophage colony-stimulating factor (GM-CSF) and whole blood superoxide anion (.O2-) generation were evaluated in 27 healthy mothers, 16 undergoing vaginal delivery (VD) and 11 elective caesarean section (ECS) and in their babies. Blood specimens were taken from the mothers at the beginning of labour, during labour, immediately after delivery and 4 days later in the VD group, and before anaesthesia, immediately after delivery and 4 days later in the ECS group; neonatal samples were taken at birth (cord blood) and 4 days later. After delivery by VD, these mothers had higher plasma levels of IL-1 beta, IL-6, IFN-gamma and higher .O2- generation than those delivered by ECS. IL-6 plasma levels and .O2- generation were higher in babies born by VD than in those born by ECS. A statistically significant correlation between IL-6 plasma levels and .O2- release was observed in cord blood of babies born by VD (r = 0.69; p < 0.006). The study demonstrates that labour plays an important role in modulating host defences in the newborn.