The biological properties of elastase inhibitor from Aspergillus flavus (AFLEI) were investigated. AFLEI was produced at the highest rate when casamino acid was used as the nitrogen source. When a mixture of AFLEI (approx. molecular weight, 7,500) and elastase from A. flavus (approx. molecular weight, 40,000) was detected using anti-AFLEI antibody, molecular weight of the detected mixture was approximately 48,000, indicating that AFLEI and elastase bound at a proportion of 1 : 1. When immunocompromised mice administrered of immunosuppressive (cyclophosphamide) were infected by inhalation of A. flavus and administered amphotericin B (AMB) alone or in combination with AFLEI, survival rate tended to be higher with combination treatment than with AMB alone. Moreover, although extensive bleeding was seen in pathology sections taken from rat lung resected 24 hr after purified elastase was administered to the lung via the bronchus, this bleeding was inhibited by AFLEI. These findings indicate that for the treatment of aspergillosis, combination of an existing antifungal agent with AFLEI can be expected to provide greater therapeutic benefits than administration of an antifungal agent alone.