Nonspecific immunity plays a major role in the clearance of the opportunistic fungal pathogen, Aspergillus fumigatus. However, the mode of recognition of Aspergillus conidia by the different elements of an intact phagocyte system remains poorly understood. This study concentrated on the initial interaction between infective conidia and human polymorphonuclear cells (PMNL) using a double-label immunofluorescent method. At 37 degrees C, association was rapid. Although ingestion rates were slower than association, both association and ingestion plateaued at 90 min and were considerably reduced in the presence of fetal calf serum. In the presence of autologous plasma, association increased as the conidia-to-cell ratio increased, but the percentage of associated conidia that were ingested decreased. At 4 degrees C, phagocytosis was negligible. Phagocytosis experiments in the presence of plasma treated in various ways to inhibit the complement pathways demonstrated that optimal association was dependent on an active alternative complement pathway.