Monocytes, peritoneal macrophages, inflammatory polykaryons, and myeloid cell lines were incubated on slices of human cortical bone and assessed for their capacity to resorb bone by scanning electron microscopy. None of these cell types, mononuclear or multinucleate, induced any detectable change in the bone surface, even after prolonged incubation, and even in the presence of macrophage activators. These findings emphasise the inadequacies of mononuclear phagocytes as surrogate osteoclasts, and expose a discrepancy between 45Ca release and bone resorption.