In 19 normal subjects an increase in the number of circulating neutrophils was observed after intramuscular injection of 1 mg glucagon. The response began at the end of the 1st hour following the injection and persisted beyond the 8th hour, with a peak between 2 and 5 hours. No response was obtained in patients with bone marrow aplasia, either primary or associated with acute leukaemia. In 20 patients with chronic primary neutropenia, the degree of response was proportional to the percentage of medullary polymorphonuclears. A comparison between the kinetics of the glucagon-induced granulocyte response and that of the response induced by other neutrophil mobilizing agents suggested that glucagon acts by releasing granulocytes from the bone marrow reserve compartment. The finding that an infusion of dibutyryl cyclic AMP results in granulocyte mobilization suggests that the effects of glucagon are mediated by cAMP at cell level. Since the glucagon response test is harmless and gives rapid and pronounced results, it may be useful in investigation of patients with neutropenia. In addition, the glucagon-induced granulocyte mobilization might improve leucocyte yield in blood donors used for transfusion in agranulocytosis.