Neutrophil-specific granule deficiency is a disorder of leukocyte maturation associated with decreased levels of mRNA for a distinct subset of granule proteins. Our work indicates that this disorder, previously thought to be limited to the neutrophil lineage, can also include eosinophils. Immunofluorescence staining led to the discovery of a small but distinct population of peripheral white blood cells containing eosinophil peroxidase (EPO). Unlike normal eosinophils, these EPO+ cells do not have large, eosin-staining cytoplasmic granules, and are indistinguishable from granule-deficient neutrophils by light microscopy. The EPO+ cell lineage did resemble the normal eosinophil lineage in its ability to respond dramatically to granulocyte-macrophage colony-stimulating factor (GM-CSF); the size of the EPO+ peripheral cell population increased approximately 70-fold over baseline in response to GM-CSF administration. The EPO+ cells contained eosinophil Charcot-Leyden crystal protein, but were deficient in three eosinophil-specific granule proteins; neither eosinophil cationic protein, eosinophil-derived neurotoxin, nor major basic protein could be detected in these EPO+ cells, despite the presence of mRNA transcripts for each of the three absent proteins.