Epidemiological studies have identified childhood exposure to environmental tobacco smoke as a significant risk factor for the onset and exacerbation of asthma, but studies of smoking in adults are less conclusive, and mainstream cigarette smoke (MCS) has been reported to both enhance and attenuate allergic airway inflammation in animal models. We sensitized mice to ovalbumin (OVA) and exposed them to MCS in a well-characterized exposure system. Exposure to MCS (600 mg/m(3) total suspended particulates, TSP) for 1 h/day suppresses the allergic airway response, with reductions in eosinophilia, tissue inflammation, goblet cell metaplasia, IL-4 and IL-5 in bronchoalveolar lavage (BAL) fluid, and OVA-specific antibodies. Suppression is associated with a loss of antigen-specific proliferation and cytokine production by T cells. However, exposure to a lower dose of MCS (77 mg/m(3) TSP) had no effect on the number of BAL eosinophils or OVA-specific antibodies. This is the first report to demonstrate, using identical smoking methodologies, that MCS inhibits immune responses in a dose-dependent manner and may explain the observation that, although smoking provokes a systemic inflammatory response, it also inhibits T cell-mediated responses involved in a number of diseases.