Osteoclasts were incubated on a glass or plastic substrate and the effect of calcitonin (CT) on their behaviour was observed. Before exposure to CT the osteoclasts were actively motile, the cytoplasm advancing behind broad pseudopodial (lamellipodial) processes which showed intense ruflling activity. CT caused cessation of lamellipodial activity within minutes, followed by gradual fragmentation and retraction of lamellipodia. Complete osteoclast quiescence was regularly induced by concentrations of CT above 50 pg/ml, and lesser degrees of quiescence were induced at concentrations down to 10 pg/ml. This quiescent state was reversed on removing CT from the medium, and was abrogated by prior treatement of osteoclasts with trypsin. The quiescent state did not reduce the longevity of the cells in culture. nor did it affect their resistance to removal from glass by trypsin. CT showed no influence on the pseudopodial activity of osteoblasts, peritoneal macrophages or inflammatory giant cells. Osteoclast quiescence seems to be a reversible state induced by the interaction of CT with a trypsin-sensitive CT receptor, present on osteoclasts. The range of concentrations which induce partial osteoclast quiescence are within the physiological range of serum concentrations in man, and this suggests that CT plays a physiological role in the regulation of osteoclast activity. The behavioural change induced by CT in osteoclasts may help to identify the precursor cell of the osteoclast and may assist investigations into the mechanism of control of osteoclasis.