Reaginic antibodies (IgE and some IgG subclasses) and mast cells play important roles in the induction of type I immediate hypersensitivity reactions. These antibodies bind through their Fc fragment to high affinity receptors (Fc epsilon RI) present in the membrane of mast cells and basophils. The cross-linking of the receptor initiates a coordinated sequence of biochemical and morphological events that results in exocytosis of secretory granules containing preformed inflammatory mediators, secretion of newly formed lipidmediators, and secretion of cytokines. Previously, several studies have investigated the role of reaginic antibodies in the pathogenesis of Recurrent Airway Obstruction (RAO). However, whereas the immunological aspects of RAO have been extensively studied, the precise sequence of events involved in the pathogenesis remains not completely understood, and the role of IgE in this disease remains controversial. Therefore, in this study, several bioassays were conducted to determine whether reaginic antibodies from RAO-affected horses have the ability to activate mast cells. These bioassays involved measuring degranulation of rat peritoneal mast cells, activation of NF-kappa B and morphological changes in basophilic leukemia cells (RBL-2H3) following incubation with horse serum from RAO-affected horses that were sensitive and insensitive to Aspergillus fumigatus (A. fumigatus) or from unaffected horses. Our results show that reaginic antibodies from horses sensitive to A. fumigatus were able to degranulate rat peritoneal mast cells. In additon, there was an increase in the activity of the transcription factor NF-kappa B in RBL-2H3 cells, and morphological changes were observed in these cells once cross-linking was produced. These findings were not found in horses not sensitive to A. fumigatus and healthy horses. These bioassays demonstrate the ability of reaginic antibodies to stimulate mast cells and indicate that these antibodies could be involved in the immunological mechanisms leading to RAO.