The effect of ricin treatment on IgE levels and airway inflammation has been examined in ovalbumin (OA)-sensitized rats. Ricin augmented the total IgE titre and caused about a 10-fold increase in the antigen-specific IgE titre. Repeat antigen challenges (every second day) with an OA aerosol increased the number of eosinophils and neutrophils in bronchoalveolar lavage fluid and produced airway hyperresponsiveness (AHR) 24 h after the third challenge. Ricin pretreatment caused a significantly larger influx of eosinophils, neutrophils and lymphocytes into lavage fluid, but there was no further increase in AHR. The present study demonstrates that ricin augments IgE production and the inflammatory response, but that AHR is unrelated to the number of infiltrating inflammatory cells.