Levels of chemiluminescence (ChL) were measured in peripheral monocytes obtained from patients with acute leukemia. Items examined were base line (BL), peak level (PL), peak time (PT) and BL/PL ratio. Levels of monocyte ChL examined during different states of acute leukemia [before chemotherapy, state of complete remission (CR) and state of relapse] showed no significant differences as compared to those in healthy controls. However, prolongation of PT in untreated cases, elevation of BL and low BL/PL ratio in cases of CR and elevation of PL in relapsed cases tended to be observed. Four of 5 acute leukemia patients with CR, in which monocyte ChL was measured sequentially, showed an elevation of BL and PL before and/or during the early stage of relapse. Moderate inverse relations were observed between the percentage of leukemic cells in the marrow and monocyte BL and PL, respectively, and between the absolute counts of monocytes in the blood and monocyte BL and PL, respectively. These results suggest that there are two dynamic phases of monocyte ChL in acute leukemia; namely, BL and PL are rather high when the volume of leukemic cells is small, and BL and PL decrease as leukemic cells enlarge. The sequential measurement of monocyte ChL in the state of CR may aid in the early recognition of relapse.