A 28-yr-old man developed asthma shortly after being exposed to nickel sulfate (NiSO4) in a metal-plating factory. Recordings of peak expiratory flow rates at work and for an off-work period showed increased variations at work, with best values on weekends. Allergy prick skin tests elicited an immediate reaction with NiSO4 at a concentration of 10 mg/ml. Experimental inhalation challenges with NiSO4 at the same concentration of 10 mg/ml for 60 sec produced a bronchial obstruction typical of an early asthmatic response. A control asthmatic subject with a similar level of sensitivity to histamine failed to react after the same inhalation of NiSO4, which suggests a specific rather than irritant mechanism for the reaction. the patient had evidence of IgE antibody to a NiSO4-human serum albumin antigen. His serum selectively bound 63 Ni, and the test was selectively blocked by nonlabeled nickel sulfate. These results, evaluated together, suggest the development of occupational asthma caused by allergy to NiSO4 in this patient.