Scedosporium apiospermum (Pseudallescheria boydii) is an emerging opportunistic filamentous fungus that causes serious infections in both immunocompetent and immunocompromised patients. To gain insight into the immunopathogenesis of infections due to S. apiospermum, the antifungal activities of human polymorphonuclear leukocytes (PMNs), mononuclear leukocytes (MNCs), and monocyte-derived macrophages (MDMs) against two clinical isolates of S. apiospermum were evaluated. Isolate SA54A was amphotericin B resistant and was the cause of a fatal disseminated infection. Isolate SA1216 (cultured from a successfully treated localized subcutaneous infection) was susceptible to amphotericin B. MDMs exhibited similar phagocytic activities against conidia of both isolates. However, PMNs and MNCs responded differently to the hyphae of these two isolates. Serum opsonization of hyphae resulted in a higher level of superoxide anion (O(2)(-)) release by PMNs in response to SA54A (amphotericin B resistant) than that seen in response to SA1216 (amphotericin B susceptible; P < 0.001). Despite this increased O(2)(-) production, PMNs and MNCs induced less hyphal damage to SA54A than to SA1216 (P < 0.001). To investigate the potential mechanisms responsible for these differences, hyphal damage was evaluated in the presence of antifungal oxidative metabolites as well as in the presence of a series of inhibitors and scavengers of antifungal PMN function. Mannose, catalase, superoxide dismutase, dimethyl sulfoxide, and heparin had no effect on PMN-induced hyphal damage to either of the two isolates. However, azide, which inhibits PMN myeloperoxidase activity, significantly reduced hyphal damage to SA1216 (P < 0.01) but not to SA54A. Hyphae of SA1216 were slightly more susceptible to oxidative pathway products, particularly HOCl, than those of SA54A. Thus, S. apiospermum is susceptible to antifungal phagocytic function to various degrees. The selective inhibitory pattern of azide with respect to hyphal damage and the parallel susceptibility to HOCl suggests an important difference in susceptibilities to myeloperoxidase products that may be related to the various levels of pathogenicity and amphotericin B resistance of S. apiospermum.