Calcitonin (CT) is a potent inhibitor of bone resorption and there are abundant CT receptors on mature osteoclasts. The relationship between osteoclast precursors and monocyte lineage is far from clear. We recently showed that human cord monocytes in culture, by contrast to adult monocytes, develop some features of osteoclast precursors. We therefore assessed the presence of CT receptors on monocytes. We could not demonstrate any CT receptors on adult monocytes. By contrast, we observed in cultured cord monocytes increased cAMP production in presence of CT. This cAMP response was observed after a 2-week culture and only in the presence of 10–9 M 1, 25-dihydroxyvitamin D. After a 3-week culture, CT 10–9 to 10–6 M increased cAMP production dose dependency from 10–9 M; however the curve was shallower than the one observed in a control CT receptor positive tumoral cell line, MCF7. 125I sCT bound specifically to cord monooytes cultured during 3 weeks; apparent dissociation constant (Kd) was 3.3 ± 2.2 10–10 M and average receptor number was 5.1 ± 0.4 104/cell. On autoradiography all the cells, whether mono or multinucleated, were labeled with 125sCT. One or 2-h exposure to salmon CT did not induce cell contraction. In conclusion, CT receptors can be induced on newborn cord monocytes in the presence of 1, 25-dihydroxyvi-tamin D. This observation shows that osteoclasts and fetal monocytes share a common membrane determinant.