Calcitonin decreased calcium uptake in specific rat bone cell populations obtained by sequential collagenase digestions of calvaria. The calcitonin effect on calcium uptake was observed in the same populations that manifested calcitonin-induced increases in cyclic AMP as well as high levels of acid phosphatase and the ability to release 45Ca from prelabeled devitalized bone. No effect of calcitonin was observed in cell populations that had high levels of alkaline phosphatase and lacked the potential to resorb devitalized bone. These results suggest that changes in cell calcium as well as cyclic AMP may be involved in the mode of action of calcitonin on osteoclast-like cells.