Penicillium marneffei produced and secreted a 38-kDa antigen that appeared to be specific for this dimorphic fungus. This component could not be detected in antigenic extracts of Histoplasma capsulatum, Cryptococcus neoformans, Aspergillus niger, Aspergillus fumigatus, Aspergillus flavus, Aspergillus terreus, Candida albicans, and two other species of Penicillium by immunoblot analysis against the sera from patients with culture-confirmed penicilliosis marneffei. Antibody reactive with this antigen was found in a large proportion of human immunodeficiency virus (HIV)-positive patients, indicating a presumptive diagnosis of P. marneffei infection. A small number of asymptomatic HIV-seropositive patients and HIV-seropositive patients with other fungal infections were also found to be positive by this analysis, suggesting that subclinical or mixed fungal infections involving P. marneffei are not uncommon.