Carbonic anhydrase was localized in osteoclasts by light and electron microscopy using a preembedding peroxidase-antiperoxidase staining method. Osteoclasts on the endosteal surface of the metatarsi of calcitonin-treated and untreated (control) chicks were studied. A positive staining reaction was seen in the cytosol, in the Golgi apparatus, in and on vesicles, on the plasma membrane of the ruffled border, and on the bone surface beneath control osteoclasts. After calcitonin treatment, positive staining reactions were seen at the same sites except that staining was absent on the plasma membrane and endosteal bone surface. The morphology of osteoclasts from calcitonin-treated chicks was indicative of cell inactivity. The carbonic anhydrase which was bound to the plasma membrane of the ruffled border is appropriately arrayed for hydrogen ion secretion and subsequent mineral dissolution. The presence of the enzyme within lysosomelike vesicles and on the endosteal surface beneath active cells suggests that it may be released into the resorbing zone along with the lysosomal hydrolases. The function of the extracellular enzyme is also unknown.