Bronchoalveolar lavage (BAL) has been performed in 22 healthy nonsmoking male volunteers after exposure to 8 ppm SO2 (20 mg/m3). The exposure level exceeds the US Short-Term Exposure Limit (STEL) of 5 ppm, but occurs as peak exposures in industrial indoor environments. Exposures were made during light work on a bicycle ergometer in an environmental exposure chamber for 20 min. BAL was performed 2 wk or more before exposure and 4, 8, 24, and 72 h after exposure in eight subjects at each time interval. Four hours after exposure significant increases were found in the numbers of lysozyme-positive macrophages, lymphocytes, and mast cells (p less than 0.02 to 0.05). Lymphocytes, lysozyme-positive macrophages, total count of alveolar macrophages, and total cell number increased to peak values 24 h after exposure (p less than 0.02 to 0.05). Seventy-two hours after exposure the cell numbers and distribution had returned to normal. The time course of the cell reactions found in BAL fluid after controlled SO2 exposure represents a new and previously not reported response to a noxious gas.