To study the various reactions of gram-negative bacteria and their endotoxins observed in workers exposed to cotton dust, experiments were undertaken where cotton mill workers carded cottons from different geographic locations, each containing different amounts of bacterial endotoxins. Exposure was determined as the vertical elutriator dust and endotoxin levels. Measurements were made of the forced expiratory volume in one second (FEV1) and the number of blood neutrophils before and after work, and the prevalence of symptoms of byssinosis was recorded. There was a significant correlation between the vertical elutriator endotoxin levels and the group mean changes in FEV1 (p less than 0.01) but no correlation between vertical elutriator dust levels and changes in FEV1. There was also a dose-response relationship between the endotoxin levels and the presence of symptoms of byssinosis in the exposed workers as well as between endotoxin levels and an increase in blood neutrophils. This observation supports findings from several previous studies and suggests that endotoxin triggers the mechanisms responsible for the decrease in respiratory function in the byssinosis syndrome. Other constituents of cotton dust could also be of importance for this reaction.