Batches of soluble antigen prepared by mechanical disintegration of Aspergillus fumigatus reproducibly contained identical protein components on polyacrylamide gels. This consistency was ascribed to standardization at the procedure for disruption: there was little variation in the protein patterns obtained from A. fumigatus mycelium grown for times varying from one to seven days at temperatures from 27 to 37 degrees C. Rabbit antisera to different A. fumigatus antigen preparations varied in their reactivity in crossed immunoelectrophoresis tests, but line immunoelectrophoresis indicated that all the antisera contained the same qualitative antibody components to the fungus. All but one of these components appeared to be present in serum samples from two patients with invasive pulmonary aspergillosis and high precipitin titres to A. fumigatus. The data suggest that many antibodies with different specificities are formed in response to aspergillosis in vivo, so that most preparations of A. fumigatus antigens are likely to contain at least one component capable of detecting antibodies to the fungus for diagnostic purposes.