Cytochrome b558, involved in superoxide generation in phagocytes, was found to be expressed on the cell surface of most normal peripheral B lymphocytes. The cytochrome was not found on the surface of peripheral T lymphocytes, natural killer cells, or peripheral lymphocytes derived from patients with X-linked chronic granulomatous disease. On stimulation, at least half of peripheral B lymphocytes could generate superoxide anion as detected by superoxide dismutase-sensitive nitroblue tetrazorium reduction. Cytochrome b558 was not present on the surface of pre-pre B cells or pre-B cells, but did appear at the early B-cell stage. It disappeared from the B-cell surface during terminal differentiation to plasma cells. The transient expression of the cytochrome in B-cell lineage may indicate that superoxide generation is important for the function of these cells at certain stages.