Although a specific assay is desirable, the Histoplasma and Blastomyces antigens are immunologically identical and current antigen detection tests cannot reliably differentiate histoplasmosis from blastomycosis. Thus, the Histoplasma antigen test can be used to diagnose either mycosis. Differentiation between histoplasmosis and blastomycosis can usually be made based on epidemiologic, clinical, histopathologic, culture, or serologic differences, but early differentiation is clinically less important since management is similar in ill patients.12 However, treatment is offered universally in blastomycosis13 and not in all patients diagnosed with histoplasmosis,14 highlighting an important drawback. Additionally, cross reactions can be expected in specimens from some patients with coccidioidomycosis, paracoccidioidomycosis, and penicilliosis marneffei,9,15 but not in specimens from patients with candidiasis,15 cryptococcosis,16 and Pneumocystis infection.8 This cross reactivity may not reduce the value of the information, however, since treatment of the endemic mycoses is similar, except that self-limited infection may be less likely in blastomycosis.13 Although not the most rapid method of diagnosis, most patients with histoplasmosis have H or M precipitin bands by immunodiffusion or positive tests for complement-fixing antibodies to H capsulatum. Some patients with blastomycosis have A precipitin bands by immunodiffusion,17 helping to distinguish the two mycoses. Additionally, methods for measuring the antibody response to the Blastomyces WI-1 antigen could assist in diagnosing blastomycosis.18 Positive results also were noted in about 8% of specimens containing high concentrations of Aspergillus galactomannan. Previously we noted that histoplasmosis was a cause for false-positive results in the Platelia Aspergillus EIA but we did not observe positive results in the Histoplasma antigen EIA in specimens from patients with aspergillosis.19 We further demonstrated that very high levels of Aspergillus galactomannan were needed to cause positive results in the Histoplasma EIA,19 making it unlikely that the positive results for Aspergillus galactomannan in this report represented cross reactions caused by histoplasmosis.