Qualitative platelet parameters (volume, 5 hydroxy-tryptamine (5 HT) organelles studied by the mepacrine test, and density) were evaluated in 31 patients with acute leukaemia: 11 myelomonocytic (AML), 8 lymphoblastic (ALL), 12 granulocytic (AGL). Macrothrombocytosis was observed in most of the cases of AML, was rare in AGL and was never found in ALL. The 5 HT organelles/volume ratio was normal in AGL and ALL but was significantly decreased in AML. In contrast, platelet density distribution was always abnormal whatever the platelet volume and 5 HT organelle concentration. Thus, using simple new methods, convenient even in cases of thrombocytopenia, we demonstrate that qualitative platelet abnormalities are a constant feature in acue leukaemia and that they are more severe in AML.