The effect of elastolytic proteinase on quantitative nitroblue tetrazolium (NBT) dye reduction and chemotaxis of human neutrophil was examined. Elastolytic proteinase is derived from Aspergillus fumigatus 9409. NBT dye reduction was inhibited significantly by 133microEg/ml of elastolytic proteinase (p<0.05). Chemotaxis was also inhibited significantly by 400microEg/ml of the enzyme, compared to the control group (p<0.05). These findings suggest that elastolytic proteinase, depending on its concentration, acts as an inhibitory agent to both NBT dye reduction and chemotaxis activities in the human neutrophil. The findings that this enzyme suppressed neutrophil function in A. fumigatus infection is of importance, because it is now suspected that the enzyme has the potential to cause infection.