Serum levels of interleukin-1 beta (IL-1 beta) in newborn infants with septicaemia were measured and possible relationships between the clinical course of the infants, causative micro-organisms and IL-1 beta levels were investigated in a prospective study. The study groups comprised 49 newborn infants (25 mature, 24 premature) with proven sepsis and 40 healthy newborn infants (20 mature, 20 premature). Serum IL-1 beta levels were measured using the IL-1 beta immunoradiometric assay. The levels were found to be lower in neonates with sepsis (median 0.1 pg/ml) than in healthy controls (median 27.9 pg/ml) (p < 0.001). Non-significant trends towards lower levels were observed in children with shock and in non-survivors. No correlation was found between IL-1 beta and postnatal age, gestational age or the study weight of the patients. There was no significant difference in the serum IL-1 beta level in septic patients infected with Gram-positive bacteria and those infected with Gram-negative bacteria. The results show that the concentration of IL-1 beta is significantly decreased in preterm and term neonates with sepsis.