We have studied changes in the mean linear fluorescence of monocyte CD14, the endotoxin receptor, in a group of 25 cotton workers (mean age 41 years). Workers were placed into one of 4 exposure categories. Monocyte CD14 fluorescence was measured by flow cytometry, on samples taken on Monday morning (CD14 pre), after 6 hours (CD14 +6) and after 72 hours (CD14 +72). Workers in the higher exposure categories 3 (between 10-100 EU/ m³) and 4 (between 100-400 EU/ m³) showed significantly increased CD14 expression after 6 hours, rising from a mean of 5.58 at baseline to 8.76 at 6 hours (p=0.01) in group 3, the corresponding figures for the highest exposure group being 6.61 and 9.82. All CD14 values returned to near baseline levels when measured after shift on the fourth working day. We propose that CD14 expression on monocytes may be used to monitor workers exposed to endotoxin and may help to determine the mechanism of action of lipopolysaccharide in producing respiratory ill health.