A complementary DNA expression library derived from marrow samples from myeloma patients was recently screened and human macrophage inflammatory protein-1α (hMIP-1α) was identified as an osteoclastogenic factor expressed in these samples. hMIP-1α 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-1α enhanced OCL formation induced by human interleukin-6 (IL-6), which is produced by marrow stromal cells when they interact with myeloma cells. hMIP-1α also enhanced OCL formation induced by parathyroid hormone-related protein (PTHrP) and receptor activator of nuclear factor κB ligand (RANKL), factors also implicated in myeloma bone disease. Time-course studies revealed that the hMIP-1α 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-1α (CCR1 and CCR5) were expressed by human bone marrow and highly purified early OCL precursors. Furthermore, hMIP-1α did not increase expression of RANKL. These data demonstrate that hMIP-1α 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-1α 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.