KC, the product of an immediate early gene induced in mouse fibroblasts by platelet-derived growth factor, was expressed in Escherichia coli by using a maltose binding protein vector and biochemically characterized as a ligand for both murine and human polymorphonuclear neutrophils (PMN). On murine PMN, KC is both a potent chemoattractant and up-regulator of Mac-1 cell surface expression. On human PMN, in contrast, KC exhibits dissociation of its chemoattractant and Mac-1 up-regulatory activities. Although KC strongly increases Mac-1 expression on human PMN, it does not induce chemotaxis in vitro. 125I-KC-Tyr binds to both mouse and human PMN with two classes of binding sites, including high affinity sites of 0.8 and 2 nM, with approximately 9,000 and 10,000 sites per cell, respectively. On mouse PMN, human macrophage inflammatory protein (MIP)-2 alpha and MIP-2 beta compete for 125I-KC-Tyr binding with high affinity, whereas the murine beta-chemokine TCA-3 does not compete. KC binds to human PMN by the IL-8 type B receptor and to murine PMN by a murine IL-8 type B receptor homologue. 125I-KC-Tyr also binds to human RBC with a single class of high affinity sites. KC mRNA is constitutively expressed in multiple murine tissues. With human IL-8 and KC cDNA as probes, a mouse neutrophil exudate library was screened: KC and MIP-2 were the dominant chemokine species found. Thus, KC appears to be intimately involved in murine inflammation and its constitutive expression may have a role in the basal trafficking of neutrophils.