Relative sensitivities of pulmonary function tests, arterial blood gas measurements, and determinations of leukocyte changes in detecting human responses to cotton dust were assessed. Subjects were 12 cotton textile workers, aged 24 to 55, with 4 to 38 years in the cotton mills. Of these workers, seven were smokers, five had symptoms of bronchitis, and seven had symptoms of byssinosis. Tests were performed before and after 2, 4, and 6 hours of heavy exposure to cotton dust and during a comparison period and included maximal expiratory flow rate (MEFR), vital capacity, forced vital capacity (FVC), forced expiratory volume (FEV) in 1 and 3 seconds, total lung capacity, white blood cell count in blood and nasal secretions, and arterial oxygen tension. Compared to comparison conditions, 6 hours of dust exposure produced significant reductions in MEFR, 1 and 3 second FEVs, and maximum flow rate at 50 percent FVC. FEV measurements were the most significant and consistent discriminators of comparison and experimental conditions, decreasing 4 to 6 percent from baseline during exposure. Decreases in arterial oxygen tension were not significant overall, but exceeded 10 percent in two subjects. Increases in blood leukocytes were significant only after 4 hours of exposure, showed large variations throughout the trials, and were not related to FEV changes. After 4 hours of exposure, the ratio of polymorphonuclear cells to epithelial cells in nasal secretions increased more than 200 percent. The authors conclude that FEV in 1 second remains a good choice as a reliable indicator of rapid airway narrowing.