Extracts of tibiae of suckling rats were prepared with 0.3 M KCl containing 0.1% Triton X-100 and were chromatographed with CM-52 cellulose. Most of the acid phosphatase activity determined with p-nitrophenylphosphate (p-NPP) was bound to the cellulose and could be eluted with a sodium acetate buffer gradient in 2 distinct peaks. The major peak, E2, was bound strongly to the cellulose and showed high activity with p-NPP and inorganic pyrophosphate (P-Pi), but only slight activity with beta-glycerophosphate (beta-GP) and was unaffected by tartrate. The minor peak, E1, was weakly bound to the adsorbent, showed equal activity with p-NPP and beta-GP, but negligible activity with P-Pi and was completely inhibited by tartrate. These results support earlier evidence suggesting that bone contains at least 2 different acid phosphatases and that the more abundant enzyme may function as a pyrophosphatase.