Improved diagnostic techniques have been needed for pulmonary aspergillosis, a common opportunistic fungal infection with a high mortality rate. Radioimmunoassay was used in this study to detect Aspergillus antigen in bronchoalveolar lavage fluid. In four patients with invasive aspergillosis or aspergillomas, Aspergillus antigen was detected in bronchoalveolar lavage fluid. In two patients, results of fungal cultures were negative or delayed. The specificity of antigen detection in bronchoalveolar lavage fluid was 91 percent in 35 control patients with a variety of pulmonary disorders. The technique or radioimmunoassay detection of microbial antigen in bronchoalveolar lavage fluid appears promising for the diagnosis of aspergillosis.