1. The Nitroblue Tetrazolium (NBT) test is a measure of the phagocytosis of particulate complexes of NBT and heparin and/or fibrinogen by neutrophil leucocytes.
2. Humoral factors in the plasma of ill patients stimulate neutrophil leucocytes of normal subjects to ingest these complexes.
3. The acute phase protein, orosomucoid and endotoxin stimulate reduction of NBT in vitro and could be responsible for some positive NBT tests in vivo.
4. The effect of factors promoting phagocytosis may be masked by an inability of neutrophil leucocytes to respond to stimulation.
5. This defective neutrophil function could result from the replacement of circulating neutrophil leucocytes by less mature, less actively phagocytic cells or by the presence of circulating immune complexes.