The capacity of human peripheral blood monocytes to generate highly reactive oxygen-derived molecules was studied during differentiation of the cells to macrophages in vitro. The effect of semipurified native interferon gamma (IFN gamma) on the differentiation-associated production of active oxygen intermediates was assessed by continuous exposure of the cells to IFN gamma or by adding it to the cultures at different stages of in vitro differentiation. Chemiluminescence (CL) response, triggered by opsonised zymosan, was highest in fresh isolated monocytes and fell constantly during a two-week culture. IFN gamma had little effect on CL. Generation of intracellular O2- was determined by the reduction of nitroblue tetrazolium (NBT). Zymosan-induced NBT reduction increased slightly during monocyte to macrophage differentiation and was further enhanced by continuous presence of IFN gamma. Hydrogen peroxide (H2O2) release, triggered by phorbol myristate acetate (PMA), was low in monocytes, increased slightly, reaching a maximum on day 3, and declined thereafter. H2O2 secretion was greatly enhanced by the presence of IFN gamma and remained raised for at least 14 d. When added at intervals to spontaneously matured monocytes, IFN gamma had only modest and transient effects on the generation of intracellular O2- and H2O2. It is concluded that IFN gamma seems so to modulate human mononuclear phagocyte differentiation that they maintain or increase their oxidative metabolic capacity.