Evidence suggests that neutrophils are important in host defenses against invasive aspergillosis and mucormycosis, although hyphae in these lesions are too large to be phagocytized. Interactions of neutrophils with hyphae of Aspergillus fumigatus and Rhizopus oryzae were studed in vitro. Light and electron microscopic observations indicated that neutrophils attached to and spread over the surfaces of hyphae, even in the absence of serum. This was followed by dramatic morphologic changes which suggested severe damage and probably death of hyphae. An assay of neutrophil-induced reduction of uptake of radioisotopes was used to quantitate damage to the fungi by neutrophils from normal subjects. Damage to hyphae was inhibited by a variety of compounds which are known to affect neutrophil surface functions, motility, and metabolism. Use of inhibitors of oxidative microbicidal mechanisms of neutrophils indicated the central importance of these mechanisms in damage to hyphae. Inhibitors of neutrophil cationic proteins altered damage only to Rhizopus. Damage to hyphae by lysozyme suggested that it may play a secondary role in the neutrophil, primarily against Aspergillus. This new nonphagocytic mechanism may play an important role in host defenses against these and other hyphal forms of fungi.