To ascertain whether progression from the chronic to the accelerated and blastic phases of chronic myeloid leukemia (CML) is associated with the loss of the myeloid differentiation antigens of the neutrophil granulocytes (NG), we analyzed two monoclonal antibodies (PMN 31D8 and PMN 13F6) recognizing normal peripheral blood NG membrane antigens in 49 patients in different evolutive stages of CML. Since five patients were studied twice, a total of 54 studies were carried out. Fourteen patients were evaluated at diagnosis, 12 in the controlled chronic phase within 1 year from diagnosis, 14 in the advanced chronic phase (median evolution 3.25 years), and 14 in the accelerated (five cases) or blastic (nine cases) phases. Fourteen normal subjects served as a control group. At diagnosis, a significant decrease in the positivity for both antigens was observed with respect to controls, probably due to the circulation of incompletely mature NG. In the early chronic phase the values were within the normal range, whereas a significant decrease was registered in the advanced chronic phase and especially in the accelerated/blastic phase. A negative correlation between the NG positivity for both markers and the time elapsed from the moment of obtaining the initial control of the disease was found, suggesting that a progressive loss of the myeloid antigens of the NG occurs during the evolutive course of CML. These results seem to confirm the usefulness of the NG myeloid differentiation antigen study as an evolutive parameter in CML.