A monoclonal antibody (MAb)-based sandwich enzyme-linked immunosorbent assay was developed for the detection of Penicillium antigen in clinical specimens from patients with Penicillium marneffei infection. The IgM from clone 8C3, reactive with both yeast and mycelial antigens, was immobilized on a microtiter plate. The antigen in serum or urine was captured and detected with biotinylated polyclonal rabbit anti-P. marneffei antibody. The test was sensitive in detecting soluble yeast exoantigen at a concentration as low as 4.88 ng/mL. The reliability of the assay method was evaluated using sera at a dilution of 1:2 from 18 patients with culture-proven penicilliosis, 23 patients with other fungal and bacterial infections, and 125 healthy volunteers. The method exhibited a sensitivity of 72%, a specificity of 100%, a positive predictive value of 100%, a negative predictive value of 97%, and an accuracy of 97%. However, the sensitivity could be increased to 90% when the specimens were used undiluted. The method was also useful for the detection of antigen secreted in the urine of the patients. The results showed that the newly described assay method can be used in the diagnosis of P. marneffei infection with a high degree of reliability.