1. The anomalous course of the curves relating K loss to time in systems containing human red cells in isotonic NaCl, and particularly the high positions of the asymptotes to which the curves apparently proceed, are due to the population of red cells consisting of at least two components, one of which loses K more readily than the other.
2. Since large K-Na exchanges can occur between red cells and an isotonic suspension medium without there being large volume changes, a restatement of the "dual mechanism of hemolysis" hypothesis, which takes account of the cell's being slowly permeable to cations, is required. If some approximations of minor consequence are allowed, the hypothesis can be restated in a quantitatively satisfactory way.
3. The general features of K-Na exchanges, including prolytic exchanges are summarized.