Pulmonary function has been studied in students and cotton mill workers in an experimental cardroom. The subjects were exposed to various levels of cotton dust over a 4-hour working period. Vertical elutriated dust levels and airborne endotoxin were determined according to previously described methods. In the first set of experiments, the correlation between dust and endotoxin levels was very high. Dose response relationships for students demonstrated a reacting threshold of about 0.5 mg/m³ of dust or 0.5 µg/m³ of endotoxin. The workers considered as a group showed a slightly lower reaction threshold than the students and the decrease in FEV(1) was larger at equivalent dust/endotoxin levels as compared to students. Among the workers, certain subjects who were smokers showed a stronger reaction than the remainder of the group. If these persons were excluded from the group the group averages for workers did not significantly differ from those of the students.