Endotoxin contamination of cotton from three geographical areas was determined. Samples obtained from California, Mississippi, and Texas were processed and dust was collected on polyvinyl-chloride filters from the breathing zone of workers exposed during 6 hours of carding operations. Cotton samples were washed at 66 degrees-C using a 50 to 1 water to fiber ratio, and dust was measured on a cotton dust weight basis. Samples were extracted and endotoxin contents of the supernatant were quantified on a spectrophotometer. Cotton from Mississippi was also treated with various temperatures and water to fiber ratios using the wool scouring washing technique, and was also fractioned aerodynamically using a cascade impactor. The endotoxin content of untreated cotton samples from California, Mississippi, and Texas was 40.3, 167.6, and 390.2 nanograms per milligram of fiber, respectively. Washing of the samples reduced the endotoxin content by 65 to 96 percent. Washing conditions affected the endotoxic content of carded cotton dust. The rayon wash line hot water treatment was the most effective by removing 95 percent of the endotoxin while cold water treatment was the least effective by only removing 83 percent of the endotoxin contamination. Wool scouring with hot water treatment removed 87 percent of the endotoxins. A similar reduction was seen in airborne samples taken from the washed and untreated carding areas and from areas where the Mississippi cotton had been treated under different washing conditions. Airborne dust collected at various stages of the cascade compactor showed that all size fractions of the airborne dust contained quantifiable endotoxin contamination with the highest content occurring in size range of 2.9 to 5.9 microns. The authors conclude that assay of endotoxins provides a reliable method for monitoring the cleanliness of cotton or airborne cotton dust.