We studied the effect of anticoagulants and incubation time on neutrophil nitroblue tetrazolium (NBT) scores. Heparin (10 IU/ml) and EDTA (1.5 mg/ml) were used, and the preparations were incubated for periods ranging from 20 to 180 min. Heparin, in contrast to EDTA, was found to be associated with cell destruction, cell aggregates and higher NBT scores; the effects were more profound with endotoxin stimulation. The duration of incubation significantly affects NBT scores in both the heparin- and EDTA-anticoagulated samples, and the effect is more marked with heparin. Cell destruction and cell clumping are both enhanced with incubation, again more so with heparin. It is concluded that when performing NBT tests, blood samples would best be anticoagulated with EDTA to ensure more consistent results.