The role of myeloperoxidase in the regulation of the respiratory burst of human neutrophils activated by the chemotactic peptide (N-formyl-L-methionyl-L-leucyl-L-phenylalanine) plus cytochalasin B was determined by using anti-(human myeloperoxidase) antibody. The respiratory burst activated under these conditions consisted of an initial (1-2 min) phase with high rates of O2 uptake, luminol-dependent chemiluminescence and superoxide radical (O2-.) generation and a second, more sustained, phase of lower magnitude of chemiluminescence and O2 uptake: O2-. generation did not occur during this second phase. In cell suspensions stimulated in the presence of anti-(human myeloperoxidase) antibody, the magnitude of the initial phase of both O2 uptake and O2-. generation was unaffected, but these high rates were maintained over much longer periods than in control suspensions. It is therefore proposed that a product of myeloperoxidase normally regulates the duration of O2-. generation during the respiratory burst, possibly by inhibition of NADPH oxidase.