We report the case of a 55-year-old male who experienced cough, dyspnea, wheezing, and nasal congestion immediately upon exposure to FD&C Blue Dye No. 2 (Indigotine) at work. The patient had worked for 10 years mixing and grinding powdered synthetic red, yellow, and blue dyes for use in foods; symptoms had occurred for 2 years and only with exposure to Indigotine (C16H8N2Na2O8S2), a free flowing blue powder. Prick testing to Indigotine (20 mg/mL) was negative. ELISA failed to detect specific IgE, IgA, IgM, or IgG to Indigotine-HSA conjugates. Bronchial challenge was done according to the method of Pepys et al. beginning with 4 x 10(-4) lactose dilution of Indigotine powder. After 5 minutes of exposure to 4 gm Indigotine/100 gm lactose, the patient developed dyspnea and audible wheezing. At 20 minutes postexposure, there was a 20% decline in FEV1 from prechallenge baseline; no late phase response was observed. A second bronchial challenge with sodium sulfate, the major nondye product additive was negative. To our knowledge, this is the first documented case of occupational asthma due to FD&C Blue Dye No. 2. The pathogenesis is uncertain but does not appear to be IgE mediated.