A sandwich ELISA has been developed, using an affinity purified monospecific antiserum as a capture antibody, to detect specific IgG and IgG sub-classes to a major antigen (Ag 7) of Aspergillus fumigatus in the sera of patients with allergic bronchopulmonary aspergillosis (ABPA). Significantly elevated levels of specific IgG to Ag 7 were detected in 97% of ABPA sera tested, as compared to control sera and to sera from A. fumigatus skin-prick test positive individuals. IgG sub-class antibody levels to Ag 7 were also determined in a similar sandwich ELISA, but using specific monoclonal antisera instead of the polyclonal anti-IgG. Both Ag 7 specific IgG1 and IgG4 levels were found to be significantly raised in the ABPA sera compared to controls. It is proposed that this antigen-specific ELISA may provide a more specific diagnostic test for IgG antibody detection in sera of ABPA patients.