In a typical case of prolymphocytic leukemia, blood smears and lymph node imprints have been investigated cytologically and cytochemically. It could be shown that many leukemic cells in both blood smears and lymph node imprints contained tartrate resistant acid phosphatase activity. Furthermore, the lymph node imprints disclosed many cells with a positive alkaline phosphatase reaction. Such a reaction hitherto has not been described in malignant cells of lymphoproliferative diseases. The cytochemical results underline that prolymphocytic leukemia indeed is a separate entity which can be differentiated from hairy cell leukemia and chronic lymphatic leukemia not only morphologically but also cytochemically. In addition, the case shows that leukemic blood cells are not inevitably identical with those occurring in organ infiltrates.