THE precipitin reaction is also known as the “biological reaction” for proteins; it enables us to distinguish between proteins by using the animal body as a test-tube, and to establish differences between them which no other form of test-tube will detect. It is best known as a means for distinguishing human from other forms of blood. The procedure is briefly to inject an animal (usually a rabbit) repeatedly with a foreign protein; the serum of that animal then gives a precipitate with that protein, but with no other. So if the material injected is human blood, a precipitate is produced when the serum of the blood of the rabbit is added to human blood, or at any rate to the blood of the group of animals (the higher apes) to which man belongs, but not with any other sort of blood. The reaction is of value in forensic medicine, and it is also of value to the zoologist, as it enables him to demonstrate the blood-relationships of animals, and by the amount of precipitate to ascertain the degree of the relationship in figures.