Previous reports suggest that byssinosis, an asthma-like condition among textile workers, may be mediated in part by histamine liberated following inhalation of dust. A simple, sensitive, and reliable procedure using pig platelets which contain the unusually high concentration of 0-8-1 6 ug histamine/109 cells has been devised for the assay of histamine-releasing factors in cotton mill dust and related materials, and has yielded results generally in accordance with earlier assays using chopped lung tissue. As little as 50-100 ,ug of total extractable substances from cotton mill dust can be measured. The activity of the extract is associated with the non-dialysable high molecular weight portion. However, conditions of acid hydrolysis do not destroy the activity. Extracts of leaves from different varieties of plant are highly potent, which suggests that the factors responsible for byssinosis are widely distributed plant components, present in textile fibre plants and converted to a respirable form by handling processes. Ellagic acid and sodium metasilicate release histamine from pig platelets, and represent new classes of compounds with possible roles in the aetiology of byssinosis.Byssinosis affects as many as a quarter of the cotton workers in the United States, and is of increasing concern to the industry (Harris et al., 1972;Bouhuys, 1974). The primary symptoms are respiratory difficulty, tightness in the chest, shortness of breath, cough, and decreased forced expiratory ventilation. The effects are most severe after absence from work (on Monday or after holidays) but gradually diminish as the work week progresses. The incidence and harshness of the attacks are related to the atmospheric concentration of respirable dust particles from the cotton materials. The disease eventually can develop into a chronic form with accompanying disablement (Harris et al., 1972;Bouhuys, 1974).Identification of the factor(s) in cotton mill dust which may be responsible for byssinosis could help to prevent or to alleviate this ailment. The nature of the symptoms and results of experimental studies suggest that induction of histamine release by cotton Requests for reprints to