To study the potential application of glucocorticosteroid administration for the measurement of the bone marrow neutrophil reserve response, blood neutrophil count changes were measured in normal subjects after the administration of intravenous hydrocortisone (25, 50, 100, 200, and 400 mg) and oral prednisone (5, 10, 20, 40, and 80 mg). The upper three doses of both steroids increased the blood neutrophil count by approximately 4,000 cells/mm3. The neutrophilia occurring after hydrocortisone (200 mg) and/or prednisone (40 mg) was compared with that observed after endotoxin (0.8 ng/kg) and etiocholanolone (0.1 mg/kg) in 14 normal subjects, 7 patients with Wegener's granulomatosis on cyclophosphamide therapy and 10 patients with chronic idiopathic neutropenia. The normal responses (mean increase of blood neutrophils/mm3 above base line +/- 1 SEM) were: hydrocortisone 4,220 +/- 320, prednisone 4,610 +/- 360, endotoxin 6,060 +/- 880, and etiocholanolone 3,780 +/- 440. In the patient studies, etiocholanolone gave the smallest mean responses, but, in general, the results were similar for all agents. These data indicate that these glucocorticosteroids can be used as equivalent agents to endotoxin and etiocholanolone for measuring the neutrophil reserve response.