Introduction It is generally true that the antibacterial properties of both normal and immune sera are greater when perfectly fresh than after being heated or allowed to stand for weeks or months with the addition of a preservative as shown in chart 7 with the opsonins in antimeningitis serum. This is probably due not only to deterioration of specific antibodies in immune serum but also to the loss of certain labile substances in normal and immune sera alike and particularly those substances generally designated as complements and normal opsonins. Antibacterial sera are especially likely to show differences in activity in their fresh state as compared with the same sera after being heated or after standing, and particularly when examined by test tube methods, whereas the antitoxic sera are usually more stabile; these differences in the case of the antibacterial sera may be ascribed in part to the loss of the labile complementary substances regarded as essential or important in the mechanism of their antibacterial activity.

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