Alendronate is a powerful therapeutic agent for the treatment of hypercalcemia in malignancy and osteoporosis and has recently been developed as a treatment for hypercalcemia of malignancy. In this study, time-lapse cinemicrography was used to investigate the effects of this agent on the morphology and the motility of human osteoclast-like multinucleated cells (MNCs) from human bone marrow. Alendronate at 10⁻⁵ M induced contraction of the cells starting 7.5 h after its addition. contraction was markedly induced immediately after alendronate removal. However, contraction almost disappeared 18 h after removal, and osteoclast-like MNCs recovered their original sizes and shape. There was only partial recovery from contraction after alendronate treatment at 10⁻⁴ M. In contrast, untreated control cells did not change their morphology after washing with culture medium. Motility analysis showed that osteoclast-like MNCs treated with 10⁻⁵ M alendronate moved actively after washing, but at 10⁻⁴ M the motility locus was very narrow. At 10⁻⁴ M, the actin ring in the cells began to break down, beginning 6 h after addition. The effects of alendronate on human osteoclast-like MNCs morphology and motility were reversible at 10⁻⁵ M, suggesting that alendronate dose not cause any cellular damages in human osteoclasts up to 10⁻⁵ M, which is an effective dose for bone resorption.