The absorption of bone in Grüneberg microphthalmic mice and the op op osteopetrotic rat can be stimulated by an injection of compatible bone marrow, and complete resorption can occur. It is probable that the bone-absorbing osteoclast responsible is derived from a haematopoietic stem cell resident in the bone marrow (BM). Maintenance of the resorption depends on the survival of donor cells which has an important implication for clinical therapy in man. No evidence was found that the thymus played a leading role in the mechanism of bone absorption in the Grüneberg microphthalmic mouse, or in the op op rat.