The accumulation of sodium fluorescein inside K562 cells in culture was investigated. It was found that the great part of the dye was bound to the enzymes inside the cell. The efficiency of the binding was found to depend upon the pH value inside the cell. This pH value was measured in the experiment. The fraction of the fluorescein bound in the cell, the dissociation constant, the concentration of binding cites in the cell and the binding energy are estimated.