Lung function and blood neutrophils were measured in 13 cotton mill workers on different days of the work week. Antigen was prepared from cotton dust and a radioallergosorbent (RAST) test made on serum and plasma. The exposure was determined as airborne dust and bacterial endotoxin using vertical elutriators and personal samplers. Reductions in FEV1 and increased blood neutrophils were found on Monday at an exposure level of 0.32 to 0.48 mg respirable dust/m3 and 0.19 to 0.28 micrograms endotoxin/m3. The changes in blood neutrophils correlated with endotoxin levels. The FEV1 decreases were present on Monday but not on Wednesday and Friday. The RAST ratios were low and not correlated with FEV1. No systematic changes in RAST ratios were present over the Monday shift. The results suggest that the importance of immunologic reactions other than type 1 should be investigated.