AbstractAcid phosphatase (AcP) in neoplastic cells from various lymphoid leukemias was examined. In the cytochemical studies, tartrate‐resistant AcP (T‐rAcP) activity was observed in the neoplastic cells from well‐differentiated lymphoid leukemias such as adult T‐cell leukemia (ATL), B‐cell chronic lymphocytic leukemia (B‐CLL), T‐cell chronic lymphocytic leukemia (T‐CLL), and hairy‐cell leukemia (HCL). T‐rAcP activity was also detected in a small number of leukemic cells obtained from T‐cell acute lymphoblastic leukemia (T‐ALL), while it was not detected in the neoplastic cells from null‐ALL, macroglobulinemia, and multiple myeloma (MM). In the electrophoretical studies, fraction 1 (F‐1), F‐3, F‐3b, and F‐4 were completely tartrate‐sensitive, while F‐2 was partially resistant and F‐5 was completely resistant. T‐rAcP activity (F‐5) was observed in ATL cells, B‐CLL cells, and HCL cells, while it was not detected in ALL cells, macroglobulinemia cells, and MM cells. The present study indicates that T‐rAcP activity is observed not only in HCL cells but also in the well‐differentiated lymphoid cells such as ATL cells, B‐CLL and T‐CLL cells except the most highly differ