To study the relationship granulocyte alkaline phosphatase (AP)--zinc (Zn) in mixed leukocyte suspensions, the enzyme from 51 mixed leukocyte suspensions was separated and purified. In the pure extract, determinations were carried out for enzymatic activity--using the autoanalyzer LKB 8600--and for Zn content--using atomic absorption spectrophotometry. From the data obtained it resulted that, in agreement with the results in the literature Zn concentration in the mixed leukocyte suspensions from chronic myeloid leukemia, (CML) patients is about half that in the normal controls (0.65 +/- 0.15 as compared with 1.24 +/- 0.15 micrograms/mg total nitrogen). Granulocyte AP separated from normal leukocyte suspensions was found to bind about 95% of the total Zn content, i.e., 1.21 +/- 0.15 micrograms/mg total nitrogen and its enzymatic activity was about 16 U/g total nitrogen. In CML patients the enzyme bound a mean Zn concentration of 0.55 +/- 0.15 micrograms/mg total nitrogen and its mean activity was 2.25 U/g total nitrogen, i.e., about 86% of the normal values found in the controls. The enzymatic activity decreased to zero after only 20 min dialysis with 8-hydroxyquinoleine-5-sulfonic acid (8HC5S) as compared with the 60-80 min in the controls. It can be concluded that the leukemic leukocyte is unable to form the metal-enzyme complex owing to the deficit of the Zn involved in the enzymatic activity.