A marked difference between enzymes and toxins is that the former may bring about a very great chemical change and still may be recovered from the mixture of substances acted on and produced, while the toxin seems to be permanently used up in its toxic action and cannot be so recovered. _Toxins seem very much like enzymes whose action is restricted to living cells._ Just as enzymes are probably produced by all kinds of cells and not by bacteria alone, so toxins are produced by other organisms. Among toxins which have been carefully studied are _ricin_, the poison of the castor oil plant (_Ricinus communis_); _abrin_ of the jequirity bean (_Abrus precatorius_); _robin_ of the common locust (_Robinia pseudacacia_); poisons of spiders, scorpions, bees, fish, snakes and salamanders. It has been stated that some enzymes are thrown out from the cell and others are retained within the cell. The same is true of toxins, hence we speak of _exo-toxins_ or toxins excreted from, and _endo-toxins_ or toxins retained within the cell. Among the pathogenic bacteria there are very few which secrete toxins when growing outside the body. _Clostridium tetani_ or lockjaw bacillus, _Corynebacterium diphtheriæ_ or the diphtheria bacillus, _Clostridium botulinum_ or a bacillus causing a type of food poisoning, _Pseudomonas pyocyanea_ or the blue pus bacillus are the most important. Other pathogenic bacteria do not secrete their toxins under the above conditions, but only give them up when the cell is disintegrated either within or outside the body. For the reason that endotoxins are therefore difficult to obtain, their characteristics have not been much studied. The description of toxins as above given is intended to apply to the _exo-toxins_ of bacteria, sometimes spoken of as _true toxins_, and to the vegetable toxins (phytotoxins) which resemble them. The snake venoms and probably most of the animal toxins (zoötoxins) are very different substances. (See Chapter XXIX.) CAUSATION OF DISEASE. This subject belongs properly in special pathogenic bacteriology. It will be sufficient to indicate that bacteria may cause disease in one or more of the following ways: (_a_) blocking circulatory vessels, either blood or lymph, directly or indirectly; (_b_) destruction of tissue; (_c_) production of non-specific poisons (ptomaines, bases, nitrites, acids, gases, etc.); (_d_) production of specific poisons (toxins). ANTIBODY FORMATION. Bacteria cause the formation of specific “antibodies” when properly introduced into animals. This must be considered as a physiological activity since it is by means of substances produced within the bacterial cell that the body cells of animals are stimulated to form antibodies. (See Chapters XXVI-XXIX.) STAINING. The reaction of bacteria to various stains is dependent on their physico-chemical structure and hence is a result of physiological processes, but is best discussed separately (Chapter XIX). CULTURAL CHARACTERISTICS.