In order to determine the relationship between the anomalies affecting two types of blood cell in myeloproliferative disorders (MPD), a functional study was performed in individuals presenting with such diseases. Thus, platelet function was investigated by means of Ivy's method for bleeding time, platelet retention to glass beads, aggregation with epinephrine and density distribution on a discontinuous sucrose gradient. Simultaneously, three granulocyte functions, i.e. capillary tube random migration, particle ingestion activity and nitroblue tetrazolium dye reduction were studied. This investigation was carried out in 47 patients presenting with chronic myeloproliferative disorders (MPD): chronic granulocytic leukemia (18 cases), polycythemia vera (18 cases), myelofibrosis (6 cases) and essential thrombocythemia (5 cases). The results of the present study indicate that functional abnormalities are more frequent and more strongly marked in platelets than in phagocytes. The tests most affected were platelet density distribution and granulocytic random migration. Simultaneous assessment of platelet and phagocytic functions, though insufficient in itself to determine the type of MPD or to appraise the prognosis of the disease, could be useful in the diagnosis of some atypical cases of myeloproliferative disorders.