Summary: When normal horse-serum was warmed to about 65°C a part of its protein became denatured with the formation of a colloidal component, C, having an electrophoretic mobility approximately that of the β-globulin. Electrophoretic analysis showed that this C component arose in the first place at the expense of the globulins but if it was produced in large amounts it adsorbed considerable quantities of albumin. The C-component precipitated on dialysis; a serum containing it could be largely cleared by centrifugation. The C-colloid precipitated in these ways could be redispersed and showed the same mobility it had had in serum. It is pointed out that in hyperimmune sera the colloid C formed at the expense of the antibody-containing T and γ-globulins. Some sera intended for therapeutic uses are pasteurized to destroy foreign viruses or bacteria. Electrophoretic investigation offers a direct way of studying antibody-damage resulting from this heating.