Signalling through the chemokine stromal derived factor (SDF)-1alpha and its receptor CXCR4 has been recognized as a key event in the migratory response of hematopoietic stem and progenitor cells (HPC). Small GTPases of the Rho/Rac family might be involved in SDF-1alpha signalling at several different levels. In the present study we report that two toxins from Clostridium species which inhibit the small GTPase Rho suppressed SDF-1alpha-induced generation of intracellular calcium transients in HPC. Chelation of intracellular Ca2+ with BAPTA or depletion of intracellular Ca2+ stores with thapsigargin demonstrated that calcium transients are essential for SDF-1alpha-induced chemotactic migration of HPC. Furthermore, transplantation of HPC pretreated with Ca2+ flux inhibitors into mice revealed a suppression of HPC homing to the bone marrow and increased levels of cells remaining in the bloodstream or circulating to the spleen. Our data indicate that the small GTPase Rho is required for the induction of Ca2+ transients in HPC, which in turn are necessary for the coordinated migratory response of HPC both in vitro and in vivo. (C) 2003 Elsevier Inc. All rights reserved.