Chemokines are important mediators of cell migration during inflammation and normal leukocyte trafficking. Inflammatory chemokines are induced in multiple cell types at sites of infection. Here, we describe a novel bovine CC chemokine, designated regakine-1, that is constitutively present at high concentrations in plasma. Cloning of its gene revealed an expected two intron/three exon organization, with a rather long first intron. In addition to a 21-residue signal peptide, the coding sequence corresponded to a 71-residue secreted protein. However, the natural regakine-1 protein missed the COOH-terminal lysine residue. Regakine-1 has only weak sequence similarity (<50% identical residues) with other animal or human chemokines. Northern blot analysis demonstrated regakine-1 RNA expression in spleen and lung. At physiological concentrations (30-100 ng/mL), natural 7.5 kDa regakine-1 stimulated gelatinase B release from neutrophils and chemoattracted immature myeloid HL-60 cells, as well as mature granulocytes. Regakine-1 was more potent on human myeloid cells than the human plasma CC chemokine hemofiltrate CC chemokine-1 (HCC-1). Moreover, regakine-1 synergized with the bacterial peptide N-formylmethionylleucylphenylalanine (fMLP), yielding a 10-fold increase in neutrophil chemotactic response above their additive effect. Regakine-1 did not compete with interleukin-8 (IL-8) for binding to neutrophils, nor did it affect fMLP-induced calcium signaling, suggesting that regakine-1 recognizes a different receptor. In view of its high constitutive plasma concentration, regakine-1 is believed to recruit myeloid cells into the circulation, whereas its synergy with other neutrophil chemoattractants suggests that it also enhances the inflammatory response to infection.