The multiple effects of interleukin-1 (IL-1) on hematopoietic cells are mainly documented in disturbed hematopoiesis, but its production and participation during constitutive hematopoiesis are still unproven. To assess the involvement of IL-1 in the regulation of steady-state hematopoiesis in vivo, we have investigated the consequences of IL-1 receptor blockade by recombinant human IL-1 receptor antagonist (rhIL-1Ra) in normal CBA/H mice treated with two i.p. injections of rhIL-1Ra (2 x 50 micrograms/mouse) seventeen and two hours before sacrifice. The cellularity, the number of granulocyte-macrophage (CFU-GM), the number of erythroid (BFU-E) progenitor cells and the percentage of these cells in S phase of the cell cycle, as well as the morphologically recognizable cells in bone marrow were estimated. In peripheral blood, hematocrit, the number and differential count of nucleated cells, the number erythrocytes and the percentage of reticulocytes were determined. IL-1Ra treatment significantly reduced the number of femoral CFU-GM and BFU-E, while all the other analyzed parameters were not different from the level obtained in control, non-treated animals. These findings show that a number of bone marrow IL-1-responsive cells were affected by the IL-1 receptor blockade, indicating that the expression of IL-1 receptors and endogenous IL-1 secretion occur as part of constitutive hematopoiesis.