Simultaneous measurement of hemolysis, the volume of the intact cells, and the K lost from the intact cells of systems containing resorcinol, sodium taurocholate, and saponin shows that the volume increases may be conspicuously small while the K losses are large, and that the volume increases are un-equal for equal K losses produced by different lysins. In higher concentrations of the same lysins, the critical volume for hemolysis is a function of the nature of the lysin and of its concentration. It is impossible to say whether these observations are compatible with the current "dual mechanism" and "colloid osmotic" hypotheses of hemolysis, in which the swelling of the cell is supposed to result from the lysin having made it cation-permeable. The difficulty to be overcome is that the theory cannot be developed to describe volume changes in finite time unless we know what assumptions to make about the mobilities of K and Na, the forces driving them into and out of the cell, etc. The experimental results do not suggest, however, that any simple set of assumptions would be satisfactory. The conditions which regulate the upper limit of the swelling, i.e. the point at which a swelling phenomenon becomes a hemolytic phenomenon, are functions of the nature of the lysin and sometimes of its concentration. They require to be specified by an independent statement, over and above any statement which may be made about the rate at which swelling occurs in the system. The simplest view of the situation is that the conditions which regulate the critical volume and those which regulate the rate of swelling are both functions, as yet undefined, of the reaction which takes place between the lysin and the structural components of the red cell.