A 5-year-old boy was first seen at the age of 11 months when he presented with growth retardation, skeletal dysmorphisms and neutropenia. Since then he has remained leukopenic except when he has pulmonary infections with a marked leukocytosis. Despite his neutropenia, marked myeloid hyperplasia was evident on marrow smear examination; many cells showed abnormally hypersegmented nuclei, with unusual shape or pyknotic nuclei. Phagocytic cells showed impaired phagocytosis, candidacidal activity, metabolic burst and chemotaxis. Moreover, the patient's serum generated less chemotactic activity than normal serum. These data indicate a selectively complex defect of the neutrophil during differentiation associated with the presence of an inhibitor of chemotactic factors in the patient's serum.