Western blotting analysis and ELISA were used to examine Dp specific IgM abs in the sera and sputa of bronchial asthmatic and non-atopic children. With Western blotting Dp specific IgM abs were detected binding diffusely to high molecular weight (greater than 65 kD) components of Dp extracts and from the intensities of the staining of the sera the asthmatic children were divided into two groups: an intensive staining and a faint staining group. The non-atopic children showed still fainter staining than the latter group. ELISA titers of Dp specific IgM abs in the intensive staining group were significantly higher than in the faint staining group and the non-atopic children (p less than 0.01). Dp specific IgM abs in sputa were also detected more intensively in asthma than in non-atopy by Western blotting. The hemolytic complement activities of the fresh sera were consumed when they were incubated with Dp extracts at 37 degrees C. The degree of consumption was significantly higher in the intensive staining group than in the faint staining group and the non-atopic children (p less than 0.05). High molecular weight fractions of Dp extracts were responsible for this complement inactivation. From these results we suspected that Dp specific IgM abs might react with high molecular weight components of Dp to induce complement activation by the classical pathway and might cause chronic inflammatory changes in the respiratory mucosa. This chronic inflammation might emphasize IgE mediated asthmatic reactions to cause late phase reactions or chronic asthma.