Sensitization to HMW allergens is a relatively common immunopathogenic factor in occupational asthma. The mechanism of sensitization is an IgE-mediated, type I reaction. High molecular weight allergen refers to proteins and polymers of organic compounds over 5 kd and usually in the 20 to 50 kd range. In most cases diagnosis requires: 1. A high index of suspicion of job-related asthma. 2. Exposure to HMW compounds and clinical findings associated with such an exposure. 3. Confirmation of sensitization by appropriate in vitro or in vivo tests. 4. Confirmation of a pathogenic role by physiologic measurements in either a natural setting or during laboratory-controlled challenges.