A complementary DNA expression library derived from marrow samples from myeloma patients was recently screened and human macrophage inflammatory protein-1␣ (hMIP-1␣) was identified as an osteoclastogenic factor expressed in these samples. hMIP-1␣ enhanced osteoclast (OCL) formation in human marrow cultures and by highly purified OCL precursors in a dose-dependent manner (5-200 pg/mL). Furthermore, hMIP-1␣ enhanced OCL formation induced by human interleukin-6 (IL-6), which is produced by marrow stromal cells when they interact with myeloma cells. hMIP-1␣ also enhanced OCL formation induced by parathyroid hormone-related protein (PTHrP) and receptor activator of nuclear factor B ligand (RANKL), factors also implicated in myeloma bone disease. Timecourse studies revealed that the hMIP-1␣ acted during the last 2 weeks of the 3-week culture period. Reverse transcription-polymerase chain reaction analysis showed that the chemokine receptors for hMIP-1␣ (CCR1 and CCR5) were expressed by human bone marrow and highly purified early OCL precursors. Furthermore, hMIP-1␣ did not increase expression of RANKL. These data demonstrate that hMIP-1␣ is an osteoclastogenic factor that appears to act directly on human OCL progenitors and acts at the later stages of OCL differentiation. These data further suggest that in patients with myeloma, MIP-1␣ produced by myeloma cells, in combination with RANKL and IL-6 that are produced by marrow stromal cells in response to myeloma cells, enhances OCL formation through their combined effects on OCL precursors. (Blood. 2001;97:3349-3353)