Four amphipathic drugs, primaquine, propranolol, chlorpromazine and tetracaine, 
were used to cause endocytosis in glucose-depleted red cells, and the relative 
reduction of membrane surfaces was measured by the toluidine blue (TB) method. 
The TB measurements correlated well with the observed electron microscopic 
alterations. In vitro blood storage studies indicated loss of cell membrane 
during storage correlated with decreasing uptake of TB by the red cells. 
Regeneration of cellular ATP with adenosine did not always restore TB uptake by 
the cells. Loss of red blood cell membrane either by exocytosis or endocytosis 
may occur during normal in vivo or in vitro ageing, and may be increased in 
pathological states or by the action of drugs.