The major specific ‘B’ antigens in aqueous extracts of pigeon excreta encompass powerful hydrolytic enzymes which cleave proteins and synthetic-ester substrates. An enriched enzyme preparation, isolated by gel filtration and ion-exchange chromatography and exhibiting appreciable hydrolytic activity, still caused precipitate formation with the sera of patients with pigeon-breeder’s lung. Subsequent analysis showed that human serum possesses negligible inhibitory activity against this enzyme product. By contrast, normal-range inhibition was noted of bovine trypsin, bacterial alkaline proteinase, or human C1 esterase. No significant differences were observed in the levels of the latter inhibitors between the sera of ill pigeon breeders and of patients with closely related pulmonary disorders of unconnected aetiology.