Postphagocytic chemiluminescence of neutrophils was evaluated in infants throughout the 1st month of life, in their mothers and healthy adults to investigate further the defective chemiluminescence of neonatal neutrophils described by earlier studies. Maternal and adult values were similar and served as controls. Abnormalities both of peak chemiluminescence and of the kinetics of light emission were detected in infant neutrophils throughout the 1st month of life. Infant responses were variable, particularly at birth, when neutrophils of some infants performed comparably to those of controls.