Using microanalytic assays various phagocytic functions of separated neutrophils from preterm neonates (mean birthweight 1,506 g, n = 13) were simultaneously studied. Adherence of neutrophils to nylon fibre was decreased in cells from preterm infants (77.1 +/- 3.1%) when compared with adult controls (86.9 +/- 2.1%, mean +/- 1 SD, p less than 0.05). In addition neutrophil chemotaxis in response to zymosan activated serum was reduced in preterm neonates (131.9 +/- 19.7, adults 166.6 +/- 11.1, p less than 0.001); directed migration towards Formyl-Methionyl-Leucyl-Phenylalanine was also decreased (preterm neonates 93.4 +/- 15, adults 111.1 +/- 16.8, p less than 0.05). Preterm infants had a higher percentage of slow moving neutrophils when compared with adults (p less than 0.001). Phagocytosis of Candida albicans was reduced in neutrophils from preterm neonates (phagocytic index: preterm neonates 41.4 +/- 12.7, adults 83 +/- 7.2). Adult neutrophils ingested more Candida per cell (p less than 0.001). Chemiluminescence, exocytosis of elastase and lactoferrin during uptake of opsonized zymosan was also reduced in neutrophils from preterm neonates. However, random migration, phagocytosis of Staphylococcus aureus and production of O2- in response to Phorbol myristate acetate or opsonized zymosan were identical in cells from either source. We conclude, that these abnormalities of neutrophils could predispose the preterm infant to serious and often overwhelming bacterial and fungal infections.