Urinary and serum colony-stimulating factor (CSF) levels were measured in 11 patients with chronic idiopathic neutropenia without infections and in 10 normal individuals. Urinary CSF output was determined using mouse marrow target cells, and serum CSF activity was assayed with human marrow target cells by the double agar layer technique. Using these methods, there was no significant difference between CSF levels of neutropenic and normal subjects. These data indicate that CSF levels are not inversely related to the blood neutrophil count in chronic idiopathic neutropenia and suggest that CSF is not a hormone regulating the blood neutrophil count in a manner analogous to the erythropoietin regulation of circulating erythrocyte levels.

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