Bronchial reactivity was measured before work among 35 female and male workers from carding, spinning, warping and weaving areas in a cotton mill. The amount of airborne dust and endotoxin was measured at the different workplaces. The reactions in all but one worker were mild but significant decreases in FEV(1) were found after administration of the highest accumulated methacholine dose used (6.4 umol). An indication of a dose response relationship was found between decreases in pulmonary function after methacholine challenge and airborne endotoxin levels.