In view of recent data demonstrating defective production of tumor necrosis factor alpha (TNF cachectin) in murine autoimmune lupus nephritis, we studied the serum levels of TNF in 22 patients with systemic lupus erythematosus (SLE), using a specific radioimmunoassay. Patients with SLE had either normal or slightly elevated levels of TNF when compared with healthy control subjects. All 5 SLE patients with concomitant infections had elevated levels of TNF; those with systemic bacterial infection had markedly raised levels (median 260 pg/ml; normal less than 40). These results show that SLE in humans is not associated with a depressed level of circulating TNF, and that in SLE patients with infection, the level of TNF in the circulation increases in a manner similar to that in other subjects.