To further investigate the neutrophil dysfunction of newborn infants, we have measured expression of the neutrophil Fc gamma receptors FcRIII and FcRII in extremely immature preterm neonates born at 24 to 32 weeks of gestation. Fc receptor expression was measured by FACS analysis of cells stained with monoclonal antibody Leu11b for FcRIII and IV-3 for FcRII. “Well” preterm neonates displayed reduced FcRIII, 51.05 +/- 2.0 (mean fluorescence channel +/- SE) when compared with term neonates, 69.24 +/- 5.5 and adult controls, 71.83 +/- 3.0. “Stressed” preterm neonates with severe respiratory distress syndrome or septicemia had a further downregulation of FcRIII, 32.67 +/- 3.0 and 35.75 +/- 1.8, respectively, associated with grossly abnormal cellular fluorescence distribution. In well preterm neonates, expression of FcRIII improved to adult levels during the first two postnatal weeks, suggesting a postnatal maturation of function. Stressed neonates had signs of partial neutrophil activation (increased Mac-1 expression and chemotactic ability), leading us to propose that the further downregulation of FcRIII may be due to receptor shedding in vivo by partially activated cells. FcRII expression was found to be equivalent to adult levels in both well preterm and stressed neonates. Reduced neutrophil FcRIII expression may provide some explanation for the reported abnormalities of phagocytosis and bacterial killing in preterm neonates.