The incidence of the blood committed granulocyte progenitor cells (CFU-C) before and after epinephrine administration has been studied in 10 normal, 16 splenomegalic and 8 splenectomized subjects through a double-layer agar culture system. A significant increase of the mean values of CFU-C per milliliter of blood has been observed after epinephrine administration in normal and in splenomegalic subjects. In splenectomized patients the baseline mean values of CFU-C per milliliter of blood were higher than those observed in the other groups of subjects, but they did not increase after epinephrine infusion. The concentration of CFU-C per 10(6) total blood leukocytes was the same in all three groups of subjects and it was not modified by epinephrine administration. Our results seem to indicate that the CFU-C are distributed in two blood compartments, the spleen representing an important section of the marginal compartment of the blood CFU-C.