The NBT reduction of granulocytes was determined in 11 patients with HTLV-III infection. The reaction was measured in granulocytes both in resting state and after stimulation with Escherichia coli, as well as with and without addition of plasma. In 5 patients with AIDS, the NBT reduction was significantly decreased in all these types of experiments, when compared with 12 healthy controls. In 6 patients with the lymphadenopathy syndrome (LAS), the NBT reduction of resting granulocytes was significantly higher than that of the controls. The findings could in general not be explained by superinfections. The elevated NBT test during the long term LAS stage reflects a release of free oxygen radicals, against which treatment might be directed.