Osteoclasts develop from haemopoietic cells of the monocyte-macrophage lineage. Osteoblasts or stromal cells are essentially involved in osteoclastogenesis through cell-cell interaction with osteoclast progenitor cells. Recent findings indicate that osteoblasts/stromal cells express osteoclast differentiation factor (ODF, also called RANKL, TRANCE and OPGL) as a membrane-associated factor in response to several osteotropic factors to support osteoclast differentiation. ODF is a new member of the tumour necrosis factor (TNF) ligand family. Osteoclast precursors, which express RANK, a TNF receptor family member, recognize ODF through cell-cell interactions with osteoblasts/stromal cells, and differentiate into osteoclasts in the presence of macrophage colony-stimulating factor (M-CSF). Osteoclastogenesis inhibitory factor (OCIF, also called OPG), is a secreted TNF receptor, which acts as a decoy receptor for ODF. ODF is responsible for inducing not only differentiation, but also activation of osteoclasts. Interleukin 1 alpha (IL-1 alpha) can be substituted for ODF in inducing the activation of osteoclasts. Recently, it was shown that mouse TNF alpha stimulated the differentiation of M-CSF-dependent mouse bone marrow macrophages into osteoclasts in the presence of M-CSF without any help of osteoblasts/stromal cells. Osteoclast formation induced by TNF alpha was inhibited by antibodies against TNF type 1 receptor (TNFR1) or TNFR2, but not by OCIF. Osteoclasts induced by TNF alpha formed resorption pits on dentine slices only in the presence of IL-1 alpha. These results demonstrate that TNF alpha stimulates osteoclast differentiation through a mechanism independent of the ODF-RANK interaction. TNF alpha and IL-1 alpha may play an important role in pathological bone resorption due to inflammation.