In general, a history of exposure to "moldy" hay, birds, or other incriminated occupational or environmental inhalants in a patient with clinical and radiologic features consistent with HSP should lead to the demonstration of serum precipitins to the suspected antigen and an established diagnosis, confirmed by avoidance of the agent involved. Occasionally, other diagnostic procedures are required. The diagnosis is often difficult in domestic exposures, such as humidification and air conditioning systems. A careful environmental history is essential, and at times the physician must inspect the patient's environment personally. In most cases, the diagnosis is established if (1) the history and physical findings and pulmonary function tests indicate an interstitial lung disease, (2) the x-ray film is consistent, (3) there is exposure to a recognized cause, and (4) there is antibody to that antigen. In other exceptional circumstances, bronchoalveolar lavage may help. Biopsy is rarely needed. Special environmental studies and identification of new antigens require research facilities. Provocation tests are research procedures, not necessary for the diagnosis, and not needed in contested workmen's compensation adjudications.