The bone marrow granulocyte reserves of nine black patients with "benign" neutropenia were estimated by measuring the maximum neutrophil increment after the administration of hydrocortisone. Thirty control subjects, including 16 black and 14 white adults, were also studied. The mean neutrophil increment in the black patients with neutropenia was significantly less than that in the control subjects. The mean increment in the black control was also significantly less than that in the white control subjects. Four of the 16 black control subjects had neutrophil counts below 2,000/microliter; if these four are excluded from the analysis, the difference between the black and white control subjects is no longer significant. These data suggest that there is a subpopulation of healthy black adults with neutrophil counts below 2,000/microliter with reduced marrow granulocyte reserves as tested by corticosteroids. Bone marrow aspirates in four of the neutropenic patients showed normal cellularity and myeloid maturation suggesting that the lower increments are due to a difference in granulocyte release rather than to a difference in granulocyte production.