We identified an EST sequence that was up-regulated during osteoclast formation in the rat. Investigating further, we cloned the cDNA from rat long bone and found it to be highly homologous to the mouse CC chemokine, CCL9, both at the nucleotide and amino acid levels. The rat CCL9 amino acid sequence is 74% identical to the mouse sequence, with an additional 11% similar amino acids. Recombinant rat CCL9 was used in chemotaxis assays of rat bone marrow cells and it was found to have a strong and dose-dependent effect. In addition, CCL9 mRNA was very highly up-regulated during osteoclast differentiation of rat bone marrow-derived mononuclear cells, increasing by over 100-fold when stimulated by colony stimulating factor-1 and the TNF superfamily member, RANKL. Together, these results establish that, like the mouse, the rat also uses CCL9 to promote the complex process of osteoclast formation.