The mixed agglutination reaction with kidney cell cultures was employed to study pathologic and normal human sera. Positive results were obtained with 29 of 88 sera from patients with viral infections and with 6 of 30 sera from patients with bacterial infections. Only 1 of 65 normal sera gave a weak reaction. An increase of the reaction titer in the course of virus disease was observed in all of 10 patients whose sera obtained early and late in the disease were tested. Reactions with cell

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... es of bovine kidney were in general much stronger than reactions with kidney cultures of other species origin. The possibility was discussed that the serum factor under study is an antibody whose production was stimulated by antigens which were exposed or created in a patient's own tissues as a result of a morbid process.