As part of a multidisciplinary longitudinal approach to assess the roles of airborne cotton dust and endotoxins in affecting the respiratory health of cotton textile workers, this study was designed to quantify the endotoxin contamination of airborne vertically elutriated and total dusts. Yarn preparation areas (opening through fine spinning) were studied at two cotton textile mills which had been studied 5 years previously in Shanghai. People's Republic of China, Filter, with vertically elutriated (VE) or total dusts were mailed to the United States and endotoxin analyses were performed for each filter in duplicate with the quantitative chromogenic modification of the Limulus amebocyte lysate assay. Dusts from all areas of the textile mills contained endotoxins. Endotoxin burdens in VE dusts from the carding area were similar in both milk while the endotoxin contamination of total dust from carding in Mill 1 was over threefold greater than that of total dust from carding in Mill 2. All other areas differed between milk in both VE and total dust endotoxin burdens. Mean endotoxin levels in VE dusts from all areas of both mills were well above the reported threshold of 90 EU/m3 for acute pulmonary function effects in humans. Comparison of selected areas of both mills from the present study with the same work areas from the previous study showed that, in general, the airborne endotoxin burden was higher than levels found 5 years ago in these two mills. The data suggest that even with reduced or unchanged gravimetric dust levels in these two cotton textile mills, airborne endotoxin levels were higher and provided an increased potential for adverse respiratory response in exposed workers.