Phospholipases are a group of esterases that are composed of two major categories, the acyl hydrolases and the phosphodiesterases. Investigations on the phospholipases of Aspergillus species have been very limited to date. Indirect evidence for extracellular phospholipase activity was demonstrated in Aspergillus fumigatus by identifying the accumulation of phospholipid breakdown products in cultures of A. fumigatus grown on lecithin using fast atom bombardment spectroscopy based on their mass ratios. On the basis of the specific degradation products found, it was predicted that A. fumigatus secretes multiple extracellular phospholipases, including phospholipase A (PLA), phospholipase B (PLB), phospholipase C (PLC), and phospholipase D (PLD). More recently, the extracellular phospholipase activity was investigated in clinical and environmental isolates of A. fumigatus. In a survey to investigate the extracellular phospholipase activity in clinical and environmental isolates of A. fumigatus, the production of extracellular phospholipase of A. fumigatus collected from different centers worldwide was compared. Pathogenicity of gene knockout strains in a pulmonary animal model are required to determine the significance of PLB activity in lung infection and disease progression. Phospholipases play an important role in the induction of cytokines in mammalian cells; for example, PLC activity from Clostridium perfringens has been reported to induce the expression of interleukin-8 (IL-8) by endothelial cells. Similarly, researchers have reported that injury of endothelial cells by phospholipases following exposure to Rickettsia conorii led to the synthesis of IL-6 and IL-8.