One hundred and thirty-six textile workers were tested with natural fibers; 97 were tested with synthetic fibers. By inhalation testing 20% were positive to both types of fibers; by nasal provocative testing 30% were positive. There was a close correlation between the size of the wheals in the prick tests and the percentage of fiber challenge tests. Fiber provocation tests and metacholine tests did not display any conformity with each other. The authors feel that synthetic fibers may act as haptens and cause Type I or IgE-mediated allergy and both natural and synthetic fibers should be considered important causes of occupational asthma among textile workers.