It is the purpose of the present paper to direct the attention of biologists to important differences in the toxicity to living cells and organisms of certain commercial brands of so-called C.P. sodium chloride which have usually been treated in the past as being more or less identical chemically. The brands in question have all been used frequently at the Marine Biological Laboratory and other scientific in stitutions in this country; and, in view of the striking differences that will be shown to exist between them, the question arises how far the work of different investigators, who have in the past used sodium chlorideofunspecifiedorigin,iscomparableand,indeed,how farmany published statements concerning the physiological properties of this salt in pure solutions may be generally true. \Vhile these questions cannot as yet be answered with entire certainty, the necessity is clearly indicated for much greater care in the future than has been exercised in the past in physiological work involving this commonest of all salts. The observations which formed the beginning of this investigation were made more or less accidentally in connection with certain unpub lished studies on the hemolytic effects of ammonium chloride on the erythrocytes of the various classes of vertebrates, particularly the fishes. In the course of these studies, controls of isotonic NaCl were used for comparison, the salt employed being that which happened at the time to be in general use at the Marine Biological Laboratory. It soon became apparent that whereas the erythrocytes of the mammals remained intact almost indefinitely in such control solutions, those of severalspeciesof fishes,among them the sea robin,the butterfish,the cunner,the tautog,themackerel,thescup and the freshwater perch, underwent destruction in times ranging from a few minutes to several hours, though falling to do so in similar solutions of KC1 or CaC12 or in properlydilutedseawater. The unique behavior of NaCl is brought out in Fig. 1, in which ar
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