A whole-blood technique was used to measure simultaneously neutrophil migration, uptake, and killing of candida in 27 premature infants of low birth weight (less than 1500 g). Neutrophil migration was consistently reduced, especially in the first two weeks of life. Phagocytosis was also reduced, particularly in the first week of life and in sick patients. Killing was usually normal, except in sick patients. The three functions were not altered when the test was performed in normal adult, rather than autologous, plasma, and the reduced migration and uptake are therefore the result of an intrinsic defect of the cell. The results clarify the previous controversy concerning neutrophil function in premature infants and provide an explanation for their increased susceptibility to infection.