Raised serum IgE levels were found in a high proportion of rats that had been exposed to tobacco smoke twice daily 5 days a week for 8 wk in a Dontenville-type smoking machine. Levels above 1 ng/ml of IgE were found in nine of 20 animals exposed to cigarette smoke and in five of 20 rats exposed to smoke from cigarettes with 1.45% phenylmethyloxidiazole added for possible protection against the effects of the smoke. None of the 20 control rats exhibited similarly increased serum IgE. Exposure to tobacco smoke did not significantly affect the serum concentrations of IgM and IgG. The development of specific IgE and IgG antibodies was also influenced by tobacco smoke exposure. Rats exposed to ovalbumin aerosol developed increased levels of IgG and IgE antibodies, whereas no effect on the development of antibody titers was found in rats immunized by the subcutaneous route. This study demonstrates that exposure to tobacco smoke increases serum IgE levels and enhances sensitization via the airways by a local effect, thus supporting the "mucosal theory of atopy."