1. Methods have been described for reducing protein S-S groups, for oxidizing protein SH groups, and for estimating protein S-S and SH groups.

2. It has been found necessary in estimating the cystine content of proteins by the Folin-Marenzi method to take into account any cysteine that may be present.

3. A method for estimating the cysteine content of proteins has been described.

4. With these methods, estimations have been made of the S-S and SH groups and of the cystine and cysteine contents of a number of proteins.

5. In a denatured, but unhydrolyzed protein, the number of S-S and SH groups is equivalent to the quantity of cystine and cysteine found in the protein after hydrolysis.

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