The beta-chemokines, MIP-1 alpha/CCL3, MIP-1 beta/CCL4, and RANTES/CCL5, play a critical role in the selective accumulation and activation of macrophages in inflamed tissues. Herein, we demonstrate that the binding of each of these beta-chemokines to their cognate receptors, CCR1 and CCR5, in either macrophages or in CCR1- or CCR5-transfected CHO cells, induced actin reorganization and the formation of lamellipodia that are characteristic of the activation of the Rho family GTPase, Rac. A dominant negative mutant of Rac. but not dominant negative mutants of RhoA or Cdc42, blocked MIP-1 alpha-induced lamellipodia formation. Moreover, this MIP-1 alpha-induced Rac activation and consequent lamellipodia formation is G(i)-and phosphoinositide-3 kinase (P13K)-mediated. Thus, Rac activation is critical for both CCR1- and CCR5-triggered signaling cascades mediating beta-chemokine-induced reorganization of the actin cytoskeleton, a process essential for effective recruitment and activation of macrophages in inflammation. (c) 2005 Elsevier Inc. All rights reserved.