A 50-yr-old mold maker developed severe asthma a few weeks after commencing work with a furan binder. Asthma recurred within hours of subsequent exposure and was confirmed by measurements every 2 hr of peak flow rate. The molds were prepared by mixing sand with a resin (containing furfuryl alcohol, paraformaldehyde, and xylene) and a catalyst (containing sulfuric acid, phosphoric acid, and butyl alcohol). Occupation-type exposure in the laboratory to the resin mixed with catalyst, and to pure furfuryl alcohol mixed with sulfuric acid or butyl alcohol, provoked late asthmatic responses and heightened nonallergic bronchial responsiveness to inhaled histamine. No changes were produced by the same exposures in an asthmatic volunteer with a similar degree of histamine bronchial responsiveness, or in the worker after exposure to resin alone and catalyst alone. Avoidance of exposure was followed by clearing of symptoms and return of histamine bronchial responsiveness towards normal. The findings identify the occurrence of specific bronchial responsiveness to volatile reaction product(s) of furfuryl alcohol following reaction with sulfuric acid or with butyl alcohol. The incidence of this problem needs investigation, especially since furan-based binder systems are replacing traditional methods.