cDNA for two new members of the human CC chemokine family was isolated based on structuraland functional similarities to leukotactin 1 (Lkn-1). One of these cDNA clones was identical to the previously isolated CC chemokine, CK♀8 (MPIF1), whereas the other is a major splicing variant, therefore named CK♀8-1. The mature proteins of CK♀8-1 and CK♀8 consisted of 116 and 99 amino acids with calculated molecular weights of 12,500 and 10,950, respectively. Both CK♀8-1 and CK♀8 were potent agonists for CCR1. These chemokines induced migration of neutrophils, monocytes, and lymphocytes, which is similar to Lkn-1. They also significantly suppressed colony formation by human bone marrow, granulocyte-macrophage, erythroid, and multipotential progenitor cells stimulated by combinations of growth factors. Lkn-1 and CK♀8-1 gene were mapped to the same bacterial artificial chromosome. To our knowledge, this is the first example showing that an alternative splicing produces two active CC chemokines from a single gene.