1. The kinetics of the movements of glucose in both directions across the surface of the human red cell were studied by optical recording (Ørskov method) of resultant cell volume changes.
2. A wide experimental variety was arranged in the relations between the several quantitative factors contributing to the glucose gradient and the volume changes expected, in order to provide a maximum variety of systematic relations between those factors and the rate of glucose transfer.
3. The kinetics were shown to follow the patterns predicted on the basis of a simple carrier system, involving formation of a highly undissociated complex between the sugar and some factor in the cell surface, provided the glucose concentrations used did not exceed about 3/4 isosmotic. Certain simple properties of this system are derived from the data.
4. At very high glucose concentrations, this system apparently gradually fails to operate; this failure is reversible upon lowering of the excessive glucose concentration.
5. An empirical correction was derived for a previously known but uncalibrated optical disturbance complicating the use of the Ørskov method with media containing appreciable concentrations of non-electrolytes.