Solutions of casein are usually sprayed over the leather in the final stage of tanning. We present the result of a thorough investigation of an atopic tannery worker with occupational asthma. Data from clinical record and metacholine challenges support the occupational source of the patient's symptoms, and positive bronchial challenge with casein clearly defines it as the specific aetiological agent. The presence of specific IgE suggests a hypersensitivity type I mechanism. In the tanning process chromium salts, paraphenylendiamine and formaldehyde have all been included as specific aetiological agents of asthma, but occupational asthma induced by inhalation of casein has not previously been reported.