The prednisone test revealed normal bone marrow reserve of neutrophils (BMR) in subjects with innocent neutropenias e.g. neutropenias not associated with any other disturbances in haemopoiesis or with increased incidence of infections, assumed to be an individual anomaly of the subject. On the other hand, diminution or absence of MBR in chronic hypoplastic neutropenias, transient post-chemotherapeutic neutropenias and neutropenias with increased destruction of neutrophils were observed. The magnitude of that diminution was not dependent of the cause of neutropenia. It is suggested, that the prednisone test (and probably tests with other BMR mobilizing agents) may serve as screening tests for exclusion (or confirmation) of relative granulopoietic insufficiency as the cause of neutropenia. Only after confirmation of reduced BMR another explanations of neutropenia should be sought using other method as muramidase level, adrenaline test or granulocyte kinetics with DF32P, 3HDFP, or 51Cr.