terms after the analogy of “agglutinin” and “agglutinogen.” In fact the specific precipitation and agglutination are strictly analogous phenomena. Precipitins act on proteins in (colloidal) _solution_ and cause them to settle out, agglutinins act on substances within cells which cells are in _suspension_ in a fluid and cause the cells to settle out. Ehrlich’s theory of the formation of precipitins is similar to that of agglutinins, and need not be repeated. Substitute the corresponding words in the theory of formation of agglutinins as above given and the theory applies. The precipitin reaction has not found much practical use in bacteriology largely because the “agglutination test” takes its place as simpler of performance and just as accurate. The reaction is, however, generally applicable to filtrates of bacterial cultures and could be used if needed. The so-called “mallease” reaction in glanders is an instance. Precipitins find their greatest usefulness in legal medicine and in food adulteration work. As was noted above, if animals, rabbits for example, are immunized with the blood of another animal (human beings) precipitins are developed which are specific for the injected blood with proper dilution. This forms an extremely valuable means of determining the _kind of blood_ present in a given spot shown by chemical and spectroscopic tests to be blood and has been adopted as a legal test in countries where such rules of procedure are applied. Similarly the test has been used to identify the different kinds of meat in sausage, and different kinds of milk in a mixture. An extract of the sausage is made and tested against the serum of an animal previously treated with extract of horse meat, or hog meat, or beef, etc., the specific precipitate occurring with the specific serum. Such reactions have been obtained where the protein to be tested was diluted 100,000 times and more. Biological relationships and differences have been detected by the reaction. Human immune serum shows no reaction with the blood of any animals except to a slight extent with that of various monkeys, most with the higher, very slight with the lower Old World and scarcely any with New World monkeys. It is a fact of theoretical interest mainly that if agglutinins and precipitins themselves be injected into an animal they will act as _antigens_ and cause the formation of _antiagglutinins_ or _antiprecipitins_, which are therefore receptors of the first order since they simply combine with these immune bodies to neutralize their action, have only a combining or haptophore group. Also if agglutinins or precipitins be heated to the proper temperature they may retain their combining power but cause no agglutination or precipitation, _i.e._, they are converted into agglutinoid or precipitinoid respectively after the analogy of toxin and toxoid.