Following the basal descriptive studies of the bronchoalveolar inflammatory cell response induced by single exposure with nitrogen dioxide (NO2) in man, it was considered important to clarify the cell response to repeated exposure with NO2. This investigation was, therefore, undertaken with bronchoalveolar lavage (BAL) 3 weeks before and 24 h after six repeated exposures with 4 ppm NO2 (7 mg.m-3) in ten healthy volunteers. The exposures were performed during 20 min and every second day. Analysis of the recovered BAL fluid demonstrated that repeated exposures to NO2 caused a lung cell response different from that reported after a single exposure. Amounts of lavaged alveolar macrophages, B-cells, and natural killer (NK)-cells were decreased and the T-helper-inducer/cytotoxic-suppressor cell ratio was altered, but there was no lymphocytosis or mastocytosis as after single exposure. Lymphocyte numbers in peripheral blood were reduced after exposure. These results suggest that repeated exposure with NO2 adversely affects the immune defence. This could contribute to the increased susceptibility to airway infections reported to be associated with NO2 exposure.