Spores of Aspergillus fumigatus can interfere with certain aspects of phagocytic cell function. In this study we have looked at the ability of human pulmonary macrophages to bind and kill spores of A. fumigatus which have been opsonized in untreated and heat-treated serum. For comparison, the non-pathogenic fungus Penicillium ochrochloron has been used. More than 60% of spores of both fungal species became cell-associated with the macrophages following incubation at 37 degrees C for 1 h. Spores of A. fumigatus opsonized in 5% pooled normal serum were significantly more resistant to killing by pulmonary macrophages than similarly opsonized spores of P. ochrochloron (p less than 0.02). However, serum which had been heated to 56 degrees C for 30 min prior to opsonization significantly increased (by approximately 80%) the ability of pulmonary macrophages to kill spores of A. fumigatus when compared with untreated sera (p less than 0.001). No such difference occurred with spores of P. ochrochloron. These unexpected observations fit with the known propensity of A. fumigatus to colonize the airways of patients with asthma and cystic fibrosis, conditions in which a protein-rich bronchial exudate characteristically occurs. Moreover, the presence of such a protective mechanism in a soil organism strongly suggests that complement-like substances may also play a role in protozoal phagocytosis. This is an area of research that does not appear to have been investigated.