Alkaline phosphatase [orthophosphoric-monoester phosphohydrolase (alkalime pH optimum), EC 3.1.3.11purified from a Burkitt lymphoma cell line (Daudi) and Molney-virus-induced murine leukemia (YAC) showed unique catalytic properties in substrate specificity and inhibition by cysteamine-S-phosphate. It migrated on polyacrylamide gel electrophoresis in a single activity band. Alkaline phosphatase with similar properties was found in several human lymphoblastoid cell lines, in chronic lymphatic leukemic cells, in organs of leukemic mice, and in sera of patients with certain lymphoproliferative disorders.The unique kinetic properties of this enzyme were established using two kinds of substrate, namely, the monoesters of orthophosphoric acid (Type I) and the S-substituted monoesters of thiophosphoric acid (Type II). The enzyme catalyzed the hydrolysis of Type