Therapies differ for invasive infection due to the various filamentous fungi. However, histopathologic identification of Aspergillus, Pseudallescheria, Fusarium, Trichosporon, dematiacious Hyphomycetes, Candida, and Zygomycetes may be confused, and microbiologic isolation is often delayed. To improve diagnosis, we examined the utility of polyclonal and monoclonal antibodies to identify Aspergillus organisms by peroxidase immunohistochemical techniques. Tissues obtained from 68 infected patients were examined. Fungal antigen was detected in all 26 cases of aspergillosis by the monoclonal immunohistochemical test. Hyphae of Aspergillus organisms exhibited strong intracellular and cell wall staining, and extracellular localization was also observed. In contrast, negative tests were observed in tissues from five cases of infection with Pseudallescheria boydii, three with Trichosporon, three with Zygomycetes, six with Candida species, and one with Curvularia, from as well as 20 cases of nonfungal pneumonia. However, cross-reactivity to Coccidioides immitis spherules was noted in three cases. Staining of Candida was observed with the polyclonal antibody but not the monoclonal reagent. This is the first immunohistochemical demonstration of aspergillus antigen with a monoclonal reagent. The study indicates that the monoclonal immunohistochemical technique can distinguish Aspergillus species from other filamentous fungi and may facilitate the clinical diagnosis of invasive aspergillosis.