The present study was undertaken to determine the most specific of the 3 serotypes of Trichosporon cutaneum responsible for summer type hypersensitivity pneumonitis and to elucidate the subclass distribution of the antibody response, which is partly dependent on the nature of the causative antigen. Western blot analysis revealed that serotype II (TIMM 1318) is the most specific antigen for summer type hypersensitivity pneumonitis. The subclass distributions of anti-Trichosporon cutaneum IgG antibodies and IgA antibodies in sera and bronchoalveolar lavage fluids (BALF) from 13 patients with summer type hypersensitivity pneumonitis and 10 normal volunteers were measured by biotin-avidin-linked immunosorbent assay. The results were as follows, (1) high levels of IgA2 antibodies were found in BALF, but minute amounts in sera, (2) IgG1 antibodies were the predominant subclass in sera and BALF, (3) three out of 13 cases showed high levels of IgG3 antibodies in sera and BALF associated with low levels of IgG1 antibodies, (4) IgA1 antibodies were found in both sera and BALF. (5) the levels of all antibody subclasses were higher in BALF than in sera. The subclass distributions suggest that the protein components of Trichosporon cutaneum are predominantly immunogen, although polysaccharides may participate as an antigen in the respiratory tract.