In explanation of the fact that the red cells of different mammals show different “fragilities”, i. e., different resistances to osmotic hemolysis, it has been pointed out that the extent dA to which the least resistant cell membrane can be stretched without there being a loss of pigment depends on the mean initial cell volume V, so that the ratio dA/V is substantially constant. This purely experimental result can be interpreted as meaning that molecules adjacent to the thin (perhaps bi-molecular) layer upon which semi-permeability depends can enter it when it is stretched, thereby allowing of a certain amount of stretching before lysis occurs (Ponder1).∗ Another well known fact is that different red cells of the same animal show different “fragilities”, so that a resistance distribution of roughly symmetrical form arises with respect to tonicity. One would expect that here too the resistance might be related to the initial cell volume, and it is with this point that this note is concerned.The volume whic...