Experimental sensitization by repeated intramuscular injection of Ascaris suum antigen (Ag-As) supplemented with Freund's incomplete and complete adjuvants was carried out in 50 BALB/c CrSl c male mice (sensitized group) for 24 weeks, and the results were compared with those in a control group of 25 mice. At the injection sites of the sensitized group, granulomatous angiitis with eosinophil infiltration was observed in all mice, and fibrinoid angiitis in only four. By light and electron microscopic examinations pulmonary granulomatous vasculitis with a few eosinophils was observed at a high frequency (80%) after 12 experimental weeks. Immunohistochemical examination revealed pulmonary vascular and perivascular infiltration of L3/T4 (CD4) positive cells, B cells, IgG and C3 positive cells in addition to activated macrophages, Thy-1 T cells, IgE positive cells, and IgM positive cells after 12 experimental weeks. There were significant increases in the eosinophil cell count of the peripheral blood, the hemagglutination titers of the sheep erythrocytes, IgE and IgM antibodies to Ag-As by ELISA and Western blotting after 8 experimental weeks. After 12 experimental weeks the IgG antibody to the Ag-As was low, but it increased significantly, and the sera showed multiple precipitation lines to the Ag-As by the Ouchterlony method. In conclusion, the pulmonary granulomatous vasculitis in this study is considered to consist of allergic reactions of type IV and probably type III based on type I.