Lysis of protoplasm, either protozoal or bacterial, by the body fluids depends on two substances: (1) amboceptor, which is generally specific, but may sometimes be common, and (2) complement, which is common with reference to antigen, but is more or less specific with reference to the source of amboceptor. This complement in man's serum is more active with specific amboceptors in man's serum than with specific amboceptors in the serum of other animals. When a foreign protoplasmic substance, protozoal or bacterial, is introduced into the tissues of man,specific amboceptors develop and may be demonstrated in the body fluids, especially in the blood-serum. These amboceptors are capable of dissolving large quantities of the same kind of protoplasm in the presence of sufficient complement, but are inactive in the absence of complement. Amboceptors are not destroyed by moderate heat (56 C) or by considerable age. Complement acts with specific amboceptors to dissolve