This study aimed to investigate the activity of the antioxidant enzyme superoxide dismutase (SOD) in the three main cell types in chronic myeloproliferative disorders (CMPD) patients, i.e. polymorphonuclear leucocytes (PMNLs), erythrocytes and thrombocytes, prior to therapy. Patients with reactive neutrophilia (RN) and healthy volunteers were included as controls. The SOD activity of PMNLs was significantly decreased in CMPD and RN patients compared with healthy volunteers, whereas the SOD activity of erythrocytes was found to be significantly increased in patients with CMPD and RN compared with healthy volunteers. There were no significant differences in the SOD activity of thrombocytes between CMPD patients, RN. patients or healthy volunteers. This study indicates that the activity of the SOD enzyme in two cell types is different in CMPD patients compared with healthy subjects. Thus, SOD activity may be altered dependent on cell type and due to specific cell function.