Location of acid phosphatase outside and inside the osteoclast was studied by electron microscopic histochemistry. Osteoclasts with a ruffled border apposed to the bone showed enzyme activity in a) membrane-limited cytoplasmic bodies of different dimensions, b) some Golgi vesicles and inner Golgi cisternae, c) vacuoles and vacuole-like profiles, d) extracellular channels and channel expansions in the ruffled border, e) cell-bone interspace. The possibility of bone degradation by lysosomal enzymes both in the cytoplasmic vacuoles and in the cell-bone interspace is discussed.