The tartrate-resistant acid phosphatase was isolated from the spleen of a patient with leukemic reticuloendotheliosis. The unique characteristic of the enzyme is its similar reactivity toward p-nitrophenylphosphate, inorganic pyrophosphate ADP, and ATP. When ATP was incubated with the enzyme, the initial products were ADP and inorganic phosphate. The kinetic data confirmed that these substrates were hydrolyzed by the same enzyme. This type of substrate specificity is clearly different from acid phosphatase and pyrophosphatases previously described in the literature.