Recent studies directed toward developing a better understanding of the molecular and cellular biology basis of monocyte-derived multinucleated giant cell formation, function, and biologic activity are presented. In addition, HIV-1-infected T-lymphocyte syncytia and the significance of adhesion molecule/ligand interactions in the formation of these syncytia are described. Interleukin-4 or interleukin-13 induction of monocyte-macrophage fusion provides a model for foreign body giant cell formation. On the other hand, interferon-gamma induction of monocyte-macrophage fusion provides a model for Langhans' giant cell formation. Variations in monocyte-macrophage adhesion and fusion to form foreign body giant cells are provided by substrates with different surface chemistries. Recent advances in osteoclast biology have identified the role of tumor necrosis factor-alpha in regulating osteoclast bone resorption and receptor-ligand interactions and signal pathways for osteoclast activation. Although foreign body giant cells, Langhans' giant cells, and osteoclasts are derived from monocytes or monocyte progenitor cells, the ways in which they are formed, whether induced by cytokines, receptors, or biologic activity, are markedly different.