Plasma lactoferrin was measured within 24 h of birth in 23 preterm infants of between 24 and 36 weeks gestation. Lactoferrin concentrations fell with decreasing gestational age whilst the incidence of subsequent infection rose. Sequential measurements on a subgroup of 10 preterm infants showed that even when initial lactoferrin concentrations were within the range for term infants, they fell during the first week. Lactoferrin concentrations in preterm babies may rise transiently, such increases often being associated with clinical signs of infection. A rise in plasma lactoferrin of 200 micrograms/L or more over a period of less than 48 h is suggestive of infection. These findings are discussed in terms of both the possible role of lactoferrin, and the clinical usefulness of the measurement.