The nitroblue tetrazolium (NBT) test was performed in 22 normal control subjects and in 101 patients with bacterial and nonbacterial illnesses. A maximum of 10% of neutrophils of healthy subjects are capable of reducing NBT dye to a black formazan derivative. Three of eight untreated patients with bacteremia had normal NBT test results. The mean NBT value for untreated bacteremic patients was 21.5%. In patients with acute bacterial infection without bacteremia, 68% had a positive NBT test, the mean NBT value being 20%. Positive NBT tests were noted in three of four patients with viral illnesses, four of five patients with Hodgkin disease, and two of four with lymphomas. The NBT test has some value in diagnosing bacterial infections; however, it may be misleading and should be interpreted in the light of the clinical picture and other laboratory data.