Abstract These further studies demonstrated that most antigens and allergens detected by allergic bronchopulmonary aspergillosis (ABPA) sera were recognized by rabbit antiserum, and a major allergenic component, Ag 3, as previously demonstrated by self-crossed radioimmunoelectrophoresis, was identified in the conventional system. Affinocrossed immunoelectrophoresis with concanavalin A indicated that for the patients with ABPA, although the major antigens contained α- d-manno- or α- d-glucopyranoside terminal residues, the major allergenic components did not. Fractionation of the salt-precipitated, protein-enriched fraction by gel filtration on Sephacryl S-200, monitored by fused-rocket immunoelectrophoresis/fused-rocket radioimmunoelectrophoresis with both the ABPA serum pool and specific rabbit antisera, revealed that most of the antigens were in a molecular weight range >43 kd, whereas the allergens spread from as low as 10 to >100 kd. The major allergen, Ag 3, was identified and demonstrated to have a molecular weight of approximately 24 kd. This component does not bind to concanavalin A and is also very heat labile.