Hypersensitivity pneumonitis was produced in guinea pigs by aerosol challenge with antigens of Micropolyspora faeni following systemic immunization with soluble antigens of the organism emulsified in Freund’s complete adjuvant. The condition was transferred to normal guinea pigs with plasma or washed lymphocytes from immunized animals. The lungs of affected animals had bronchopneumonia with an accompanying interstitial reaction including vasculitis of septal capillaries, proliferation of alveolar epithelial cells, parenchymal eosinophilia and neutrophilia. The pneumonitis was considered the result of both type III (Arthus) and type IV (delayed hypersensitivity) reactions.