A group of 252 workers (176 women and 76 men) employed in 2 wool mills was studied. The mean age was 36 yr; mean exposure, 11 yr. All women were nonsmokers, and 47% of the men were regular smokers. Ventilatory function was measured by recording maximal expiratory flow-volume curves and forced expiratory volume in 1 s on the 1st working day of the week (Monday) before and after the work shift. On maximal expiratory flow-volume curves the flow rates at 50% of the control vital capacity were read. Workers exposed to wool dust for more than 10 yr had a higher prevalence of chronic respiratory symptoms than did those with less than 10 yr exposure, but the difference was not significant. Significant reductions during the work shift were found in maximal expiratory flow rates at 50% of the control vital capacity and 1 s forced expiratory volume, the first test being considerably more sensitive. Workers exposed to wool dust for more than 10 yr had significantly lower than predicted pre-shift values for maximal expiratory flow rates at 50% of the control vital capacity. Inhalation of wool dust extract caused a significant decrease of maximal expiratory flow rates at 40% 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. Preventive measures, especially medical supervision, are necessary in wool-processing mills to protect workers sensitive to dust.