1. Erythrocyte ghosts from human blood were produced by gentle water hemolysis. The ghost-containing hemolysate (about 20 mN) was added to media of different composition (KCl, NaCl, glucose, sucrose, etc.) and varying concentration ranging from 8 to 840 mN. The volume changes of the ghost cells were followed by a light absorption method. The potassium and sodium concentrations were also analyzed in some representative cases.

2. The ghosts shrank, or swelled, in two stages. An initial phase with a momentary expulsion, or uptake, of water leading to an osmotic equilibrium, was followed by a second phase in which a slow swelling or shrinking proceeded toward a final constant volume.

3. The ghosts were semipermeable in the sense that water always passed rapidly in either direction so as to maintain isotonicity with the external medium.

The relation between ghost cell volumes (V) and the total concentration (Ce) of the suspension medium can be expressed by a modified van't Hoff-Mariotte law: (Ce + a)(Vb) = constant. Here a is a term correcting for an internal pressure and b is the non-solvent volume of the ghost cells. This means that the ghosts behave as perfect osmometers.

4. On the other hand appreciable concentration differences of the K and Na ions could be maintained across the intact ghost cell membranes for long periods. Whether this phenomenon is due simply to very low cation permeability or to active transport processes cannot be decided, although the first assumption appears more probable.

5. When the ghosts were treated with small concentrations of a lytic substance like Na oleate, the alkali ion transfer was greatly increased. This seems to be a simple exchange diffusion process with simultaneous, continued maintenance of osmotic equilibrium (= the second phase).

A simplified theory is also given for the kinetics of the volume variations and ion exchange during the second phase (cf. the Appendix).

6. Miscellaneous observations on the effects of pH, and of some other substances are discussed. Some shape transformations of the ghost cells are also described.

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