In this study, assessment by a flow cytometric method using dichlorhydroxy fluorescin diacetate (DCFADH) in vitro revealed that human peripheral blood inhibits the production of active oxygen species by human peripheral neutrophils. It was also revealed that among the blood components, the plasma fraction inhibits active oxygen production most strongly. This plasma inhibitory activity was dose-dependent. Human serum also exerted an inhibitory activity; however, its activity was only one-third that of plasma. Moreover, when HL-60 human promyelocytic leukemic cells, with or without differentiation into the neutrophils by culturing with dimethyl sulfoxide (DMSO), active oxygen, which was also inhibited by plasma, was produced. Heat inactivation of the plasma did not alter the inhibitory activity, and gel filtration analysis showed that the peak activity was associated with a molecular mass of 70,000. The results of this study indicate that human plasma contains one or more substances that inhibit the active oxygen production of neutrophils, which may play an important role in inhibiting unneeded neutrophil activation in the bloodstream.