Modified cytochemical methods were used to show dipeptidyl aminopeptidases (DAP) II and IV in peripheral blood buffy coat preparations and bone marrow smears. In 23 normal buffy coats both enzymes were confined to lymphocytes. DAP II was found in T and B lymphocytes (about 80%) while DAP IV was restricted to T lymphocytes only (around 46%). In 11 normal bone marrows DAP II was found in 53% of the lymphocytes, as well as in plasma cells, macrophages, and occasional myeloblasts. DAP IV was found only in lymphocytes (around 32%). DAP II activity, but not DAP IV activity, was present in all of the mast cells in a case of systemic mastocytosis. Whereas DAP II was found, to a variable extent, in leukaemic myeloblasts, monoblasts, proerythroblasts, and in megakaryoblasts in 52 cases of acute myeloid leukaemia, DAP IV was not shown. Variable positivity to DAP II and DAP IV was found in the lymphoblasts in seven cases of acute lymphoblastic leukaemia, in 14 cases of B chronic lymphocytic leukaemia, and in three cases of non-Hodgkin's lymphoma. DAP II activity was variable compared with DAP IV activity, which was constantly reduced. Virtually all of the myeloma cells (96%) all of the myeloma cells (96%) in five cases of multiple myeloma and two cases of plasma cell leukaemia were DAP II positive and DAP IV negative. In 10 cases of hairy cell leukaemia most hairy cells were positive to DAP II (74%) with no demonstrable DAP IV activity. In a single case of Sézary's syndrome around 90% of the helper T cells were positive to DAP II with no DAP IV activity.
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