Anti-Aspergillus fumigatus alkaline protease IgG was investigated by Western blot. Specific IgG antibodies were detected in sera of two of nine patients with proven or highly probable invasive aspergillosis and in sera of one of eight patients with proven or highly probable allergic bronchopulmonary aspergillosis. No response was obtained in sera of 23 control patients. The specific but transient IgG response to this induced enzyme does not recommend its use as sole serodiagnostic aid.