Neutrophil myeloperoxidase (MPO) activity was analysed semi-quantitatively both by (i) MPO-scoring of polymorphonuclear leucocytes (PMN) and (ii) counting the MPO-deficient PMN (PMN lacking MPO) in 164 subjects (60 cases of leukaemia and 104 normal humans). The scoring method showed that 10 out of 21 (48%) cases of acute myeloid leukaemia (AML), 2 out of 10 (20%) cases of chronic myeloid leukaemia (CML), 0 out of 29 cases of lymphoid leukaemia (ALL + CLL), and 1 out of 104 normal humans had decreased MPO scores. These figures correlated well with the more simple counting of PMN lacking MPO in the same groups: 8 out of 21 (37%) cases of AML, 6 out of 10 (60%) cases of CML and 0 out of 29 cases of lymphoid leukaemia showing more than 4% PMN lacking MPO. In cases of otherwise unclassifiable acute leukaemia, a decreased MPO score and an increased number of MPO-deficient PMN suggests the diagnosis of AML and not ALL. Counting the number of PMN lacking MPO was found to be a time-saving and even more reliable method than the semiquantitative scoring of MPO activity in PMN.