Erythrocytes treated with 1-fluoro-2,4-dinitrobenzene become permeable to Na⁺ and K⁺, but not to small water-soluble nonelectrolytes hemoglobin, and eventually lyse inisotonic buffer. Erythrocytes treated with 1,5-difluoro-2,4-dinitrobenzene become permeable to Na⁺ and K⁺ but do not lyse in buffer or in water, even after extraction with lipid solvents. The difluoro compound crosslinks the membrane and increases its strength. Both reagents appear to remove the positive fixed charge responsible for the cation impermeability of the normal cell.