Aspergillosis has become the biggest cause of death among mycoses in Japan. The main causative agent is Aspergillus fumigatus and its virulence factors have been variously investigated, yet much remains to be clarified. We recently found that the culture filtrate of A. fumigatus has potent anti-leukocyte activity. It seriously damages macrophages and hampers the function of polymorphonuclear neutrophils. The filtrate is also active in vivo: when injected intraperitoneally, the culture filtrate lowered the survival of mice infected with A. fumigatus. This activity becomes overt within 24 hours of culture of the fungus. Although the mechanism of pathogenicity of A. fumigatus is believed to be somewhat complex, we suggest the activities shown in the culture filtrate may play a crucial role in the development of the infection.