Hypersensitivity pneumonitis and bronchial asthma were experimentally developed with adjuvant and aerosolized antigens used together and separately. Histological investigations of the lung were performed in several conditions depending on the use of adjuvant or antigens. The results were as follows. 1) In animals treated with BCG only, pulmonary histological findings such as granulomatous changes, small clusters of mononuclear cell accumulation, interstitial mononuclear cell infiltration and septal thickening were observed. Granulomatous changes in the condition consisted of predominant epithelioid cells. 2) In animals sensitized by aerosolized antigens only, marked histological changes such as cell infiltration around the alveolar septum and bronchiolar wall were observed in the use of Candida antigen. On the other hand, granuloma formation and cell infiltration around vascular wall were predominantly found in the use of Aspergillus antigens. 3) The incidence of granuloma formation and the degree of mononuclear cell infiltration tended to increase with the use of high concentration of antigen. 4) Histological findings such as granulomatous changes and cell infiltration were marked in the animals sensitized by both adjuvant and antigens.