7inhibits hemolytic activity. The rate of hemol ysis is influenced by temperature ( 1 1 ) as well as concentration of the mercurial reagent. Benesch et d ( 5 ) found a striking variation in the reactivity of the -SH groups of crystalline hemoglobin among different species. Those of dogs and man were less reactive than those of sheep. Racker (6) suggested that the difference in rate of reactivity between sulfhydryl groups and reagents may be influenced by properties of adjacent groups on the protein molecule.That -SH groups are necessary for the integrity of the erythrocyte is a fact. The thiols responsible for this effect are not known nor is the mechanism known. A recent report (5) indicates there are 8 moles of -SH per mole of hemoglobin in man. Previous studies have demonstrated the presence of ergothioneine ( 7 -9 ) and glutathione ( 10) within the erythrocyte. The glutathicme is in a dynamic state(3) and may traverse the cell envelope. Less is known of the role of ergothimwine.The necessity of -SH groups for the integrity of the erythrocyte suggests that control of the thiol system may be advantageous in the preservation of whole blood stored for transfusion.Conclusions. 1. The sulfhydryl reagent, p-chloromercuribenzoic acid, causes hemolysis of human erythrocytes. This action can be prevented by prior reaction with sulfhy-dry1 groups. 2. The hemolytic action of PCMB is reversible by washing cells for as long as 30 minutes afster contact with this agent. Reversibility of hemolysis lasts up to 60 minutes after addition of free sulfhydryl group. 3. No protection from PCMB hemolysis was noted with stroma whereas hemoglobin solution was effective. 4. The reaction between PCMB and the -SH group of erythrocytes is relatively slow. 5. A physiologic concentration of glucose offered no protection from hemolysis in this system.

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