The experiments presented in this paper were designed to characterize the colony-stimulating activity (CSA) from white blood cell feeder layers on colony formation by peripheral blood CFCs in soft agar double-layer cultures. The data confirm earlier observations that mononuclear cells are essential for CSA production but document the importance of neutrophilic granulocytes to enhance colony formation. Possible differences between colony enhancement by granulocytes and red cell hemolysate are discussed.

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