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2. The nunmber of reactive SH groups/molecule in the haemoglobins studied approximate either to 2 or to 4. 3. The behaviour of the reactive SH groups differs from that of cysteine in three ways: (a) the rate of reaction of heavy-metal ions with the reactive SH groups is slower than with cysteine; (b) thiosulphate interferes with the reaction between Ag+ ions and the reactive SH groups of human haemoglobin, but not with cysteine; (c) the acid-base-titration curves of human haemoglobin in the presence and absence of phenylmercuric hydroxide suggest that the reactive SH groups are not ionized at pH values less than 10.

SYNTHESIS OF L-ASCORBIC ACID 25 uronic amide and L-gulonic amide are not converted into L-ascorbic acid by the microsomes. 4. Reduced glutathione and some metal-binding agents enhance the microsomal synthesis of Lascorbic acid by stimulating the synthesis and not indirectly by protecting the synthesized ascorbic acid. 5. L-Gulonolactone and L-xylohexulonolactone have been identified as the intermediates in the conversion of D-glucuronolactone into L-ascorbic acid in the presence of cyanide by the soluble enzyme preparations from the liver of rat and goat. 6. Both D-glucuronolactone reductase and Lgulonolactone oxidase are absent from the tissues of the primates, guinea pig, Indian fruit bat and red-vented bulbul-species which cannot synthesize ascorbic acid. Our best thanks are due to Professor M. Ishidate, Tokyo, for gifts of ethyl D-glucuronate and D-glucuronic amide, and to the Indian Council of Medical Research for financing this work.

Summary Plasma membranes isolated from liver of rats given a single high dose of carbon tetrachloride were compared with controls, with respect to activity of three enzymes, 5′‐mononucleotidase, Na+‐K+‐ATPase and Mg2+‐ATPase. All three enzymes showed decreases in activity in the period 2½‐18 h following dosing with CCl4. This is considered as direct evidence that biochemical changes arise in the liver‐cell plasma membrane at an early stage after poisoning of rats with this hepatotoxin.