Workers from cotton mills were exposed to cotton dust during carding in an experimental cardroom. Cotton from different geographical locations with varying amounts of endotoxin were used. Exposure levels ranged from 0.6 to 3.6 mg dust/m3 (from a vertical elutriator) and from 0.1 to 8.0 micrograms/m3 of endotoxin. No relationship was found between the decrease in FEV1 over the workshift and the amount of airborne dust. Airborne endotoxin correlated with the decrease in FEV1 and the increase in blood neutrophils. The FEV1 decrease was more pronounced among smokers. The data suggest that the amount of airborne endotoxin determines the risk for development of the acute symptoms in the byssinosis syndrome.