In humans and rabbits two similar IL-8R mediate the chemotaxis and activation of neutrophils induced by alpha chemokines. We present data to suggest that there is only one such IL-8R gene in mice. We then use mice with a targeted deletion of this gene to characterize alpha chemokine ligands that signal via the mouse IL-8R. These experiments show that mouse macrophage inflammatory protein 2 binds the receptor with high affinity (Kd = approximately 1.5 nM) and potently activates both an intracellular Ca2+ flux and a chemotactic response, events absent in neutrophils from receptor-deleted mice. Mouse KC is approximately 10-fold less potent. These results show that macrophage inflammatory protein 2 and KC potently activate mouse neutrophils via a unique IL-8R, and these proteins may function as the major proinflammatory alpha chemokines in mice.