Purified house dust allergen has been employed for screening the susceptibility to inactivation of haemolytic complement in the blood serum of atopic individuals and control subjects. Though the atopic sera displayed a tendency to higher susceptibility, no practical distinction was possible from the sera of patients with various forms of chronic pulmonary disease, extrinsic allergic alveolitis, or urticaria. CH50 and C4 levels were unrelated to allergen-sensitivity of haemolytic complement, nor was there a correlation with the titers of naturally-occurring sheep red cell haemolysins. Although blood group specificity within the ABO system was distributed over the patients as expected for the normal population, the titers of A- and B-type antibodies displayed a trend to correlate with the allergen-sensitivity of fluid phase complement.