- The effect of hydrocortisone (HC) on the formation of granulocyte-macrophage (GM) colonies was assessed in vitro in cultures of normal human bone marrow. Subphysiological concentrations (less than or equal to 10(-8) mol/l) of HC had no impact on GM colony formation, but at physiological and therapeutic levels (10(-7) mol/l-10(-5) mol/l) the hormone stimulated the production of an increased number of colonies. By contrast, physiological concentrations of progesterone (10(-9) mol/l-10(-7) mol/l) were without effect and even at 10(-6) mol/l there was no influence of progesterone alone on GM colony formation. However, progesterone at 10(-6) mol/l inhibited the stimulant effect of HC to a degree which was related inversely to the concentration of HC and the duration of the culture interval. The data from this study suggest that the neutrophilia which accompanies glucocorticosteroid therapy is matched by an increase in the production of granulocytes and that HC may play a rôle, which is mediated by steroid receptors, in the physiological regulation of human granulopoiesis.