Adherence of neutrophils to glomerular basement membrane containing immunoglobulin G aggregates was accompanied by a marked increase in oxygen uptake (eightfold). Very little of the O2 consumed was recovered as superoxide, measured by cytochrome c reduction, or as H2O2, measured with horseradish peroxidase and scopoletin. When neutrophils were incubated with the basement membrane preparation in the presence of cerium chloride to detect H2O2, electron micrographs showed cerium perhydroxide deposits in the contact area between the cells and the basement membrane, but not on the remainder of the cell surface. The results imply that superoxide is produced only where the plasma membrane is in contact with the basement membrane matrix, and that it mostly breaks down to H2O2 or undergoes other reactions at this site. The longer lifetime of H2O2 compared with that of superoxide allows some of the H2O2 produced to be detected in the medium. The results also suggest that the area of contact between the neutrophil and surfaces such as basement membrane is inaccessible to proteins in the medium, eg, cytochrome c. Circulating scavengers such as superoxide dismutase or catalase, or proteolytic inhibitors, may therefore be unable to control events occurring at this site.