Twenty-two patients with allergic bronchopulmonary aspergillosis have been treated and evaluated for a period ranging from two months to nine years. Twelve are available for periodic blood sampling. Data are presented suggesting the following: (1) Patients with allergic bronchopulmonary aspergillosis have high initial levels of total serum immunoglobulin E (IgE). These levels are significantly higher in those patients who had not been previously treated with prednisone for the control of asthma. (2) Treatment with alternate day prednisone (0.5 mg/kg given on alternate days) caused clinical and roentgenologic improvement as well as marked decreases in total serum IgE but does not necessarily prevent recurrence of the disease. (3) Total and specific IgE against Aspergillus antigen may increase prior to and during exacerbations of disease. (4) Specific IgG and IgE against Aspergillus antigen are increased in most patients with allergic bronchopulmonary aspergillosis and reflect disease activity as evidenced by x-ray and clinical exacerbations in some cases. (5) Lymphocyte transformation using Aspergillis antigen, as measured by the whole blood technic, is present in patients with allergic bronchopulmonary aspergillosis, but serial changes in lymphocyte transformation do not correlate with disease activity. (6) Precipitin reactions are present in patients with allergic bronchopulmonary aspergillosis, but presence or absence does not reflect disease activity in most cases.