Using postmitotic granulocytes (PMGs) with low neutrophil alkaline phosphatase activity (NAP activity), factor(s) having the capacity to increase their NAP activity were examined in vitro. A high activity of the factor was demonstrated in the cystic fluid of a human squamous cell carcinoma, which is known to produce a large amount of granulocyte- macrophage colony-stimulating factor (GM-CSF). The NAP-stimulating factor increased NAP values both in PMGs from normal bone marrow and PMGs from patients with chronic myeloid leukemia (CML), and NAP values in cells treated with the factor approached or rose above those of normal peripheral granulocytes after 48 hr of culture. The effect of the factor was specific in that the factor caused stimulation only in granulocytic series. These findings may indicate that increases in NAP activity reflect maturation or granulocytes and that low NAP activity of neutrophils derived from patients with CML is due to the immaturity of these cells. The relationship between the factor responsible for the increase in NAP activity and GM-CSF is also discussed.