The paper presents experimental data showing that the complement system plays an important, underestimated so far a role in regulating hematopoiesis. Activation products of the complement C3 fragment governs the response of early hematopoietic cells to a gradient of SDF-1, which is the main stem cell chemoattractant. The complement system is activated in the bone marrow, both in the preparation myeloablative transplantation of hematopoietic cells before and during their pharmacologically induced mobilization from the bone marrow to peripheral blood.