Within a few minutes after addition to L929 cells, tumour necrosis factor-alpha (TNF alpha) induced an increase in lucigenin-enhanced chemiluminescence that could be inhibited by superoxide dismutase. The generation of superoxide anion (O2.-) was sensitive to treatment with rotenone, antimycin A and cyanide, indicating that the signal originated from mitochondria. The mechanism of production of O2.- was shown to be independent of ATP synthesis, as uncoupling of this event from mitochondrial electron transport did not alter the generation of O2.- induced by TNF alpha. Chemiluminescence was further dependent on the presence of extracellular calcium, suggesting a role for this cation as a second messenger. This hypothesis was supported by the finding that inhibition of mitochondrial calcium uptake by Ruthenium Red exerted a protective effect on TNF alpha-treated L929 cells. Increased O2.- generation was followed by a marked decrease in mitochondrial dehydrogenase activity and cellular ATP levels, while cell membrane permeability was moderately increased. A role for mitochondrial O2.- generation in TNF alpha cytotoxicity was further supported by the finding that resistant L929 cells had decreased ability to produce O2.- in response to TNF alpha. In addition, we detected a decreased activity of the mitochondrial enzyme succinate dehydrogenase in these cells, suggesting that this component of the respiratory chain might be an important contributor to the TNF alpha-induced generation of O2.-.