Bone marrow cells from 8 Ph1-positive patients with chronic myelocytic leukemia (CML) have been cultured in vitro. Cellular growth in the presence of the Ph1-chromosome and neutrophil alkaline phosphatase (NAP) have been evaluated at intervals of time during 19 days of culture. The mean viable neutrophil count reached a peak level on day 8, and the Ph1-chromosome was present till day 10 suggesting an active proliferation of leukemic stem cells. NAP activity reached a maximun level in the same number of days as the neutrophil peak.Our data suggest that newly formed neutrophils (Ph1+) in CML are normal as far as NAP activity is concerned. Moreover, they emphasize the hypothesis that in CML there is an accumulation of aged NAP-negative cells, diluting positive neutrophils with a consequent low NAP score.