Immunohistochemical localization of cathepsins B, D and L in the osteoclasts of rat alveolar and femoral bones was investigated by using the avidin-biotin-peroxidase complex method for semithin, 1-micron-thick cryosections. Extracellular immunoreactivity for cathepsins B and L was clearly demonstrated along the bone resorption lacunae; the intensity of the extracellular immunoreactivity of cathepsin L was stronger than that of cathepsin B. However, the intracellular immunoreactivity of both cathepsins was weak compared with that of cathepsin D. The intracellular immunoreactivity of cathepsin D in the osteoclasts was clearly observed in the granules and/or vacuoles, but extracellular cathepsin D immunoreactivity was either negligible or not detected along the resorption lacunae. In the adjacent sections stained with anti-cathepsin L or D, extensive extracellular deposition of cathepsin L was found along the bone resorption lacunae, with or without osteoclasts, although the intracellular reactivity of cathepsin L was weak. This is the first morphological study in which cathepsins B and L have been demonstrated to be produced in the osteoclasts and extensively secreted into resorption lacunae, and in which cathepsin D was found to be present in the cells but scantily secreted into the lacunae. These findings suggest that cathepsins B and L directly and effectively participate in the degradation of the bone matrix.