Although endotoxin has been shown to be an important determinant of the toxicity of inhaled cotton dust in experimental studies, there has been limited documentation of endotoxin concentrations to which workers are exposed in the cotton industry. Two different groups of vertical elutriator dust filters were assayed for endotoxin. One group (n = 563) was obtained from an environmental survey of 35 U.S. gins. The other group (n = 595) were obtained as part of routine environmental surveillance conducted at 6 yarn production mills, which were selected for study based on a priori knowledge of the geographic areas of growth for the cotton they process. Results from both groups were consistent with prior reports. In general, dust generated by processing cotton fiber from the San Joaquin Valley demonstrated less endotoxin contamination than dust from cotton grown in other major areas of the U.S. Cotton Belt. The observed variable contamination of airborne dust by endotoxin may have important implications regarding respiratory health effects experienced by workers exposed to cotton dust.