The erythrocytes from Morris Hepatoma 5123 bearing rats took up Na+ and K+ ions from the incubation medium and released Na into the extracellular space at lower rates than did erythrocytes from intact control rats* The lipid composition of erythrocytes membranes from the tumor-bearing rats differed from that of membranes from unaffected rats, showing increased contents of phospholipid phosphorus and a decreased content of cholesterol, resulting in decreased choleste- rol: phospholipid molar ratios.