By the immunofluorescent assay for the anti-HTLV-Ⅰ antibody, the serum of many patients with diffuse panbronchiolitis (DBP) and idiopathic interstitial pneumonia (IIP) show two types of cytoplasmic fluorescence. One is a granular pattern in the MT-1 cell and diffuse pattern in the MT-2 cell. This is the typical pattern for the anti-HTLV-Ⅰ antibody. The other is a diffuse pattern in the MT-1 and/or MT-2 cells. This pattern has been designated as HTLV-Ⅰ related reaction. These serum reactions were investigated by Western blot analysis with an MT-2 cell lysate as antigen. Although about 20% of the patients with DBP and IIP were considered to be suffering from HTLV-Ⅰ associated bronchiolo-alveolar disorder (HABA) because they showed the typical pattern of anti-HTLV-Ⅰ antibody, only those patients with HABA were shown to have the antibodies against HTLV-Ⅰ specific proteins (p19, p24, p28, pr53 and gp68). On the other hand, the patients with HTLV-Ⅰ related reaction had neither the antibodies against any HTLV-Ⅰ proteins nor the other protein components of MT-2. Thus, Western blot analysis revealed that antibodies against HTLV-Ⅰ specific proteins were present only in the patients with HABA.