cDNA for a new member of the human CC chemo-kine family was isolated and named leukotactin 1 (Lkn-1). Lkn-1, along with murine macrophage inflammatory protein-related proteins 1 and 2 (Mu CIO and MIP-l7/MRP-2). defines a subgroup of CC chemokines based on two conserved cysteines in addition to the four others conserved in all CC chemokines. The putative mature Lkn-1 is composed of 92 amino acids with a calculated molecular weight of 10,160. The Lkn-1 gene was mapped to human chromosome 17, region q 12. Recombinant Lkn-1 was a potent chemoattractant for neutrophils, monocytes, and lymphocytes and induced calcium flux in these cells. Lkn-1 specifically induced transient calcium mobilization in CCR1- and CCR3-expressing HOS cells. Lkn-1 suppressed colony formation by human bone marrow, granulocyte-macrophage, erythroid. and multipotential progenitor cells stimulated by combinations of growth factors.