Stimulation of bone resorption in organ culture by parathyroid hormone or vitamin D metabolites was blocked when colchicine was added to the medium at concentrations of 10-5- 10-7M. Colchicine inhibition was not readily reversible, as treatment for 6 hr at 10-5M blocked PTH responsiveness for 48 hr. However, 48-hr treatment with colchicine did not impair incorporation of labeled thymidine, uridine or leucine in bone cultures. Colchicine could also block resorption acutely in bones pretreated with parathyroid hormone, an effect similar to that of calcitonin. (Endocrinology92: 556, 1973)