Both lysosomal cysteine-proteinases and collagenase appear to be necessary for the resorption of actively growing, immature woven bone, but their relative roles are not yet clearly elucidated. The present evidence indicates that, during bone resorption, the osteoclast first solubilizes the mineral by a secretion of acid and then removes the exposed demineralized collagen by the action of secreted lysosomal collagenolytic cysteine-proteinases. Collagenase in bone seems to be mainly a product of osteoblasts and related cells, not osteoclasts. Its role could be limited in the removal of any non-mineralized collagen layers which could be covering mineralized bone surfaces and which seem to prevent the activation of osteoclasts and thus their action; such a "shield" of unmineralized osteoid is well-established at the surface of actively growing woven bone, although not on the resorbing surfaces of mature lamellar bone. Moreover, some osteoblast-derived procollagenase is stored in the mineralized bone matrix from which it can be released by demineralization. It is therefore possible that it may also contribute to the degradation of demineralized bone collagen once it has been released and activated by lysosomal cysteine-proteinases under the osteoclast.