ANTISTREPTOCOCCUS SERUM is the serum obtained from an animal (usually a horse) which has been repeatedly in jected with streptococcus. Such serum is prepared for use in the prevention and treatment of streptococcus infections in man, and is valuable on account of the antibodies contained in it which have been produced by the animal against the streptococcus and its poisons. This micro-organism is a very common one and is responsible for a great variety of diseases in man. However, antistreptococcus serum has been found to be beneficial in only two of these, namely, scarlet fever and erysipelas. Its value in the former disease has been definitely established by clinical experience, and in the case of the latter most reports indicate that it is often of benefit if given early in the disease. The results of its use in puerperal fever and septicaemia have, on the other hand, been disappointing. The streptococcus of scarlet fever and that of erysipelas belong to different strains, as is shown by failure of erysipelas anti-serum to be of any value in scarlet fever and vice versa.