A rapid immunoelectrophoretic assay was developed to detect antibodies to Aspergillus fumigatus catalase. The method's diagnostic sensitivity for pulmonary aspergillosis was 88% (72-97%, 95% confidence limits) in 33 patients presenting with either aspergilloma or Aspergillus lung infiltrate. The diagnostic specificity was 94% (90-97%) as judged from 191 patients with other infiltrative lung diseases, including infections and neoplasia. None of the 185 healthy subjects had catalase antibodies. The highest titres (ranging up to 256) were found in aspergillosis patients with cavitary lesions. Catalase antibody titres increased in two patients with concomitant development of lung cavities and mycetomas. In patients with resected or stable Aspergillus lung disorders catalase antibody titres declined by less than one dilution step per year.