In considering the kinetics of the simpler hæmolytic systems, we have already dealt at some length with one form of inhibition—that shown by serum in conjunction with saponin or one of the bile salts as a lysin (1, 2). This form of inhibition is that which has been most widely studied, and is one of great importance, for it has been shown that the inhibitory properties of serum are possessed by hæmoglobin, and that these properties must be taken into account when dealing with the percentage hæmolysis curves for such a substance as saponin (3). Other substances than serum, however, inhibit the action of saponin and the bile salts, and besides these, there is the class of substances wnich produce an acceleration of hæmolysis; no quantitative study of these has yet been made, and since we cannot proceed farther with the kinetics of simple hæmolytic systems without a knowledge of the way in which these accelerating and inhibiting substances produce their effect, this paper will be devoted to a quantitative study of some of the reactions involved. The phenomena to be studied fall into two classes. In the first we have those substances which produce an inhibition of hæmolysis, such as certain bases and sugars. The inhibitory action of members of this group is quite different from that shown by serum. In the second group are those substances which accelerate hæmolysis, such as most acids. One other class of acceleration can be added—that produced by serum when added in a certain manner to a hæmolytic system containing one of the bile salts; this phenomenon, however, we do not propose to consider in this paper, as its investigation presents special difficulties.