Abstract
The binding of transferrin by reticulocytes was studied at 3.degree. C and 37.degree. C. Of the total number of receptors 50% are in an exposed position on the outer side of the plasma membrane and 50% of them are in a masked position. All of the receptors can take either of the positions. The transition between these positions is temperature dependent and can be observed as the progressive phase of the transferrin binding. The transferrin receptors apparently are in dimers, 1 of the subunits being in the exposed position the other in the masked position. The subunits change position simultaneously. This movement of the receptors is independent of the presence of transferrin. In the light of these results a new model for transferrin-cell cycle is proposed.

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