Neutrophil myeloperoxidase (MPO) activity was analyzed by a semi-quantitative cytochemical method in 268 subjects divided into several groups. 17 subjects with significantly reduced MPO activity were found: 11 of 23 in the preleukemia group, 2/14 AMLs, 1/20 myeloproliferative syndrome, 1/7 carcinoma with bone marrow metastases, 1/33 diabetes mellitus and 1/50 normals. Only in the preleukemia group, was MPO significantly reduced in comparison to the normal group (p < 0.005). The high frequency of acquired MPO deficiency in preleukemia represents a useful criterium for this diagnosis. Furthermore, in these patients, as well as in the other subjects studied, no apparent correlation between MPO level and infection could be demonstrated.