6 cotton spinning mills and man made fibre spinning mills in Lancashire were studied with measurement of personal exposure to endotoxin, as part of a 5 year longitudinal study. Prevalence of byssinosis, other work-related respiratory symptoms and chronic bronchitis were estimated in 36 work areas within the 8 mills. 120 workers wore personal (Casella L/min) samplers with micro-glass fibre filters. Extraction was performed using pyrogen free water. A LAL 5000 analyser was used to batch sample the filters and unexposed control filters. Endotoxin exposure varied from 3,600 ng/m³ in cotton opening rooms to less than 30 ng/m³ in man made fibre mills. There was a high correlation between byssinosis and endotoxin levels (R=0.51), and with all work related respiratory symptoms (R=0.38), but no correlation with chronic bronchitis.