Fifty nine cotton spinners, asymptomatic on a modified MRC Respiratory Questionnaire performed histamine bronchial challenge testing by the Yan technique at the start and the end of the first and the fourth working shifts of the week. A dose response function for each of the four bronchial challenge tests in each operative was calculated by dividing the maximum fall in FEV(1) by the dose of histamine causing that fall. Smokers (n=30) and ex-smokers (n=19) demonstrated a higher dose response to histamine compared to nonsmokers (n=10)(p<0.05). The trend across the week in all three smoking groups demonstrated a small increase in dose response across both working shifts, being more marked across the fourth shift of the week. The largest across shift changes in dose response occurred in the non-smokers, with current smokers demonstrating the smallest across shift responses.