Studies of the respiratory burst in myeloperoxidase (MPO) deficient monocytes were undertaken to assess the physiologic consequence of the absence of MPO in these cells. As previously demonstrated with neutrophils, MPO-deficient monocytes had a greater initial rate, duration, and total superoxide production in response to phagocytosis of zymosan than did normal monocytes. Introduction of purified eosinophil peroxidase (EPO) into the phagosome by binding the enzyme to the surface of the zymosan particles changed the hypermetabolic characteristics of superoxide production in MPO-deficient cells to more closely resemble normal cells, but had no effect on superoxide generation by the normal monocytes. Further, inactivation of the bound EPO before ingestion restored the supranormal respiratory burst by the MPO-deficient cells. Iodination by MPO-deficient monocytes was significantly depressed as compared to normal monocytes following the ingestion of zymosan (1.9 versus 10.1 nmole I-/10(7) monocytes/30 min; p less than 0.01). In contrast, iodination was markedly augmented in MPO-deficient cells compared to normal cells after ingestion of zymosan coated with EPO (208 versus 70 nmole I-/10(7) monocytes/30 min; p less than 0.005), presumably reflecting the greater amounts of hydrogen peroxide formed by MPO-deficient cells. There were no differences in the levels of endogenous scavengers of reactive oxygen products (catalase, superoxide dismutase, glutathione peroxidase and reductase, and total glutathione) in MPO-deficient and normal monocytes that would account for the enhanced respiratory burst of MPO-deficient cells. These findings support a role for peroxidase in the termination of the respiratory burst of monocytes.