Immunoglobulin A forms the specific immune barrier of mucosal surfaces against microorganisms and potential allergens. Immunochemical studies of nasopharyngeal secretions (NPS) from 97 children showed enzymatic degradation of IgA in 18.6% of the samples. The observed fragments are characteristically released from IgA by the activity of specific IgA 1 proteases produced by certain bacterial species. A significantly higher prevalence (P less than 0.001) of IgA cleavage was observed in NPS from children with atopic diseases (61.5%) than in controls (11.9%). These results indicate that bacterium-induced local defects in the mucosal immune barrier of the upper respiratory tract may be a factor in the pathogenesis of some forms of atopic disease.