A glycoprotein antigen with an apparent mw of 45 kD was observed to be predominant in the circulating immune complexes isolated from patients of allergic bronchopulmonary aspergillosis as well as in the immune complexes prepared in vitro. Further characterisation of 45 kD antigen with trypsin showed four immunologically active peptides with mw 43, 36, 33 and 16 kD. Carbohydrate characterization using various lectins (Maackia amurensis agglutinin, Sambucus nigra agglutinin, Peanut agglutinin, Galanthus nivalis agglutinin and Datura stramonium agglutinin) showed presence of both N-linked and O-linked sugar moieties such as mannose, glucose, galactose and N-acetyl glucosamine. Predominant presence of 45 kD antigen in immune complexes, recognition of 45 kD by monoclonal antibodies raised against glycoprotein rich fraction of A. fumigatus and presence of elastinolytic protease activity indicate that 45 kD antigen is probably a potent virulence factor and may be contributing to the pathogenesis of ABPA by its biological as well as immunological activities.