In the practical production of vaccine virus calves are vaccinated much as human beings are vaccinated, but over a larger area. Usually the posterior abdomen and the insides of the thighs are covered with superficial linear incisions, and into these incisions the seed virus is rubbed. In the laboratory of the New York City Health Department all operations relating to the vaccination of the animals and to the collection of the virus are carried on in an operating-room provided with a cement floor, glazed brick walls, and equipped with enamelled metal operating furniture, such as would be used in a hospital. The attendants wear sterile gowns and the technique of the operations is aseptic. The seed virus is either humanized virus collected by touching sterile pieces of bone to the serum exuding from ruptured vesicles on the arms of children, or in the great majority of cases bovine glycerinated virus which has been preserved two months or longer. It is found that the crust of the vesicle, the serum issuing from the vesicle after the crust is removed, the pulp which forms the semi-solid contents and base of the vesicle, and the serum which exudes from the base of the vesicle after the pulp has been removed by a curette, all convey material capable of producing the vaccine vesicle in a susceptible person, and are therefore all different forms of vaccine virus. It has been shown, however, that if any of this material is filtered, so that all the solid particles are removed, the filtrate is inefficient. In other words, the serum is efficient as vaccine virus simply by virtue of the solid particles which it contains. It is also found that the pulp is so rich in the active principle of vaccine virus that it may be mixed with several times its weight of glycerin or other diluent and still maintain its efficiency. The different sorts of vaccine virus on the market are simply different ways of supplying this material coming from the vesicle. Most material is in one of three forms,-- (_a_) The pulp diluted with some excipient, such as glycerin, vaseline, or lanolin. The emulsion, made by mixture with glycerin, may be contained in a vial or in a capillary tube, or may rest on some holder, such as an ivory or bone point. In the latter case the point is usually protected by some form of cap. Mixture with vaseline or lanolin makes a paste, which is usually issued in a box. This is in use in parts of Italy and in India. (_b_) The serum dried on a holder, as an ivory or bone point or a quill. (_c_) The serum mixed with some excipient, usually solid or semi-solid, until it becomes a paste, and furnished like dried serum on a holder.