1. Quantitative precipitin studies indicate that progressive photo-oxidation progressively destroys the antigenic function of egg albumin. 2. Quantitative precipitin reactions of antisera (anti-egg albumin rabbit serum and antipneumococcus Type I horse serum) demonstrate that progressive photo-oxidation causes progressive lowering of the potency of the sera. 3. Quantitative precipitin reactions of the photo-oxidized globulin gamma fraction of anti-egg albumin rabbit serum and of Felton solution of antipneumococcus Type I horse serum show that these specific antibody fractions behave similarly to antibodies in whole sera. 4. Egg albumin whose precipitin reaction is destroyed by photo-oxidation no longer causes anaphylaxis in guinea pigs and does not produce precipitins in rabbits. 5. Chemical studies of progressively photo-oxidized egg albumin show a progressive destruction of tryptophane and histidine while tyrosine remains intact and cystine is reversibly oxidized. Sulfhydryl groups can no longer be demonstrated in photo-oxidized egg albumin whose antigenic characteristics are greatly weakened. 6. Similar studies on the globulin gamma fraction of anti-egg albumin rabbit serum and on Felton solution show no diminution of these amino acids in photo-oxidized material whose antigenic properties are destroyed. 7. The non-coagulable nitrogen and the amino nitrogen of egg albumin, antisera, and their specific antibody fractions show but an insignificant increase during photo-oxidation, indicating that the loss of the precipitin reaction is not due to splitting of the respective protein molecules. 8. Electrophoretic studies of egg albumin, antisera, and their specific antibody fractions show that photo-oxidation causes a marked alteration of the pattern of these substrates. 9. Photo-oxidation of proteins causes the formation of aggregates, indicating denaturation. 10. Hematoporphyrin migrates with the albumin fraction of unaltered as well as the photo-oxidized anti-egg albumin rabbit serum and pneumococcus Type I horse serum; in isolated proteins such as egg albumin, globulin gamma, or Felton solution, etc., the dye moves independently of the protein; after progressive photo-oxidation Hp becomes progressively fixed to the protein. Eosin behaves similarly to hematoporphyrin.