1. Hemolyzed rabbit cells contain a factor which lyses human erythrocytes in vitro when Mg++ and certain nucleotides are supplied to the system. Of the nucleotides tested ATP is the most active, although no net loss of ATP or of labile phosphate seems to be associated with the hemolytic process.

2. The lytic factor appears to be a sulfhydryl enzyme which attacks the membrane of the human red cell, its hemolytic activity being inhibited by human stroma.

3. The system is activated by glutathione and is inhibited by heavy metals, oxidized glutathione, cysteine, ergothionine, and a number of metabolic inhibitors. Physostigmine has no effect.

4. Partial purification of the lytic factor has been achieved by fractional centrifugation.

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