Neutropenia is a life-threatening sequel of hematological disorders and a dominant factor limiting the dosage of cytotoxic chemotherapy. The role of the neutrophil is of such importance in defence against microbial invasion that measures that modify the behaviour of residual hemopoietic tissue to promote a modest increase in neutrophils, can confer considerable benefit by reducing the frequency and severity of infection. Such a change can be mediated in bone marrow depression by diversion of more progeny of immature precursors into the neutrophil series, or by enhancement of the stimulatory drive operating on neutrophil production. The former effect can be achieved by hypertransfusion of red cells to reduce the demand on the limited precursor population for cells of the erythroid series. The latter effect can be achieved by administration of lithium carbonate. Neutropenia caused by autoimmune injury to the neutrophil series can also be successfully modified by measures which suppress the underlying immune dyscrasia or the function of the reticulo-endothelial system. Corticosteroid administration and splenectomy can be helpful in certain specific types of neutropenia. Administration of cyclophosphamide and azathioprine has both mutagenic and marrow suppressive potential, but can induce remissions in severe chronic isolated neutropenia and in systemic lupus erythematosis.