Previous studies have shown that spores of Aspergillus fumigatus inhibit phagocytosis and killing by macrophages and polymorphonuclear leucocytes. In order to identify the mechanisms of this interference with host defences, we have examined the effects of A. fumigatus spore diffusates on phagocytic cell function. For comparison, we have used spore diffusates of the non-pathogenic fungus Penicillium ochrochloron. The diffusates of A. fumigatus reduced the number of human polymorphonuclear leukocytes migrating towards a known chemoattractant by approximately 50% (p less than 0.001). In addition spore diffusates of A. fumigatus significantly decreased (p less than 0.001) the capacity of primed mouse peritoneal exudate cells to spread on glass. Spore diffusates of P. ochrochloron showed no comparable inhibitory effects. These studies have shown that spore diffusates of A. fumigatus inhibit the movement of the phagocytic cell membrane and are thus able to interfere with a primary function of phagocytic cells.