This paper deals with the kinetics of hemolysis in systems containing anti-sheep ambo-ceptor, sheep cells and guinea-pig complement, when three components are added in this order. If the quantity of amboceptor is kept constant, the relation between quantity of complement used and the time required for complete hemolysis in any given system is described by expressions identical with those for the action of a simple hemolysin. The equation describing the funda mental reaction between the complement and the cells, contains 3 constants, n, x, and k; the value of n, upon which the order of the reaction depends, is usually very nearly unity and the value of x and k are functions of the quantity of amboceptor used. If this equation is solved simultaneously with another expression which gives the frequency distribution of the resistance of the cells to the lysin, percentage hemolysis curves are obtained exactly as though the complement were a simple hemolysin. It is shown that amboceptor is rapidly absorbed by the cells of the system, the quantity which is taken up being, within the experimental range at least, a constant fraction of the quantity initially introduced. In the presence of a constant amount of amboceptor, complement is also absorbed by the cells, and again the amount so absorbed in a constant fraction of the quantity initially introduced. The taking up of complement by the sensitized cells occurs rapidly, and is complete within the first tenth of the time required for the hemolytic reaction to be complete. The numerical value of the fraction absorbed (indicated by {zeta]) is a function of the quantity of amboceptor used for sensi-tization: when the latter is great, a large fraction (e.g., [zeta]=0.5) of the complement is absorbed, whereas if the latter is small, the fraction absorbed is small (e.g., [zeta]=0.1). If the quantity of complement initially introduced is C and the quantity of complement absorbed at or near the cells is C, C is thus equal to [zeta]C. Similarly, the quantity of lysin actually transformed in producing lysis of the cells is x and thus must be equal to i[zeta]x. It is shown by experiment that this latter quantity is a constant, i.e., that in the lysis of a given number of cells a constant quantity of complement is used up irrespective of the amount of amboceptor used for sensitisation. The only effect which a variation in the quantity of amboceptor appears to produce is a variation in the ability of the cells to concentrate complement from the surrounding medium. With a small amount of amboceptor, [zeta] is small and a small fraction of complement is absorbed, while with a large amount of amboceptor, [zeta] is large and large amounts of complement are absorbed. Absorption of a small fraction from a relatively concentrated solution of complement, and absorption of a large fraction from a relatively dilute solution of complement thus result in the same thing, i.e., the absorption of a constant quantity and the transformation of a constant quantity x in the production of the ensuing lysis. The system containing amboceptor, cells and complement can thus be treated as a system containing a simple hemolysin, complement, which, after allowance is made for the degree to which it is absorbed at or near the cell interfaces, acts very similarly to saponin and other simple lysins.