D e l i a r t m e n t of Physiology and Biochemistry, Rutyers Uiaivemity, and tlze Dcpwtnient of Biology, Princeton Unieeisity FOUR FIGTiRES A nuniher of investigators have studied the action of sucll metabolic poisons a s KaF, I S A , SaN, and NaCK on manliiialian er~throcytes. Tilbranclt ( '3'7) observed that N a F and IAA increased tlie osmotic resistancc of red cells. I n later work ('35, '39) he came to the conclusion that the action of tliese two conipounds was primarily on the cell surface and only indirectly on glycolysis. Davson and Danielli ( '38) fount1 negligible potassium losses from rabbit red cells exposed f o r short time periods to NaF and NaCN. Davson ('41) subseqiieiitlp obtained appreciable K losses from such cells exposed to N a F for longer times. TIe interpreted his results to inean that a metabolite was foimed from the action of the poison which rendered a cell surface pernieable to I<. Harris ( '41 ) niid Danowski ( '41), both working with human red cells, formed the opinion from KaF' action on I< loss that inhibition of glycolysis, with reduction of energy for maintenance of tlie I< gradient, accounted for the loss. Ponder ('49) has recently examined the effect of WaF, TAB and NaCN on human red cells without arriving a t definitp coiiclnsioiis alllioug:.li he suggests the possibility of injury effects. F l y i n and ;?hizels ( ',X) and llaizels ( '51) report evidence whicli they intci.-This n o r k has been supported in part by a grant from the John and Mary R. Markle Foundation and the IIigyns Fund of Princeton University.