Inhalation of enzyme dusts can cause occupational asthma, and the list of enzymes able to induce asthma is increasing. We report two patients with asthma induced by occupational exposure to cellulase powder derived from Aspergillus niger. A type I hypersensitivity to this enzyme was demonstrated by means of skin test reactivity, positive passive transfer test, positive reverse enzyme immunoassay for specific IgE, and immediate bronchial provocation test response to cellulase dust. Skin tests with an A. niger extract were positive. Cross-reactivity between cellulase dust and an entire A. niger extract was also demonstrated.