There is epidemiological evidence for an increased prevalence of occupational asthma among radiographers. However, the causes of darkroom disease are not yet defined. A 37-year-old female radiographer reported work-related asthma approximately 2 yrs after starting work in a local hospital. She was atopic and showed bronchial hyperresponsiveness to methacholine. Occupational-type exposure with a fixing agent, but not with a developer, produced an immediate-type asthmatic reaction. As the fixing agent contained sodium metabisulphite (SMBS), a substance known to cause asthma, bronchial challenges with SMBS were performed in the patient and nine asthmatic controls. The patient showed a positive bronchial immediate reaction on 2 separate days after inhalation of 48 and 96 microg SMBS, and one control also showed a significant fall in forced expiratory volume in one second after inhalation of 12 microg SMBS. The positive reaction in the control subject argues for a greater susceptibility of both persons to SMBS or its reaction product sulphur dioxide, rather than for a new occupational allergen. It is concluded that sodium metabisulphite exposure should be recognised as a cause of darkroom asthma.