A group of 252 workers (176 women and 76 men) employed in two wool mills was studied. The mean age of 36 years; mean exposure, 11 years. All women were nonsmokers, and 47 per cent of the men were regular smokers. Ventilatory function was measured by recording maximal expiratory flow-volume curves and forced expiratory volume in 1 sec on the first working day of the week (Monday) before and after the work shift. On maximal expiratory flow-volume curves the flow rates at 50 per cent of the control vital capacity were read. Workers exposed to wool dust for more than 10 years had a higher prevalence of chronic respiratory symptoms than did those with less than 10 years' exposure, but the difference was not significant. Significant reductions during the work shift were found in maximal expiratory flow rates at 50 per cent of the control vital capapity and 1-sec forced expiratory volume, the first test being considerably more sensitive. Workers exposed to wool dust for more than 10 years had signficantly lower than predicted pre-shift values for maximal expiratory flow rates at 50 per cent of the control vital capapcity. Inhalation of wool dust extract caused a significant decrease of maximal expiratory flow rates at 40 per cent of the control vital capacity on partial expiratory flow-volume curves during the 100 min after exposure. Comparison with the same concentration of cotton dust extract revealed a similar effect during the first 40 min after exposure but a significantly larger effect of the cotton dust extract after 40 min. The data suggest that preventive measures, especially medical supervision, are necessary in wool-processing mills to protect workers sensitive to dust.