A study was made of variation of dust and microbial exposure across the working week in a Lancashire cotton spinning mill. Daily dust samples were collected for days 1,2,3 and 5 of the working week. Samples were taken both during morning and afternoon shifts. 3 continuous weeks of monitoring were performed at 6 sites in 4 work areas. Gram positive bacteria and fungi were measured on the morning and afternoon shifts of day 1 and were repeated on the am shift of days 3 and 5, for 2 weeks at 4 of the sites. Dust levels as measured by work area samplers for total dust ranged from 0.22mg/m3 to 3.04mg/m3. There was little in the way of variation of dust exposure within areas, except during cleaning/blowing down of machinery. There was a tendency for levels to be higher in the morning and as the week progressed, the latter reaching significance within the carding and spinning areas. There was no evidence of increased dust levels on a Monday to account for the periodicity of byssinosis. Similarly microbial levels showed no trends of increasing or decreasing exposure during the working week and were surprisingly consistent. Variation in dust or microbial exposure, is not sufficient to explain the unusual periodicity of byssinosis.