We have studied the sulfhydryl groups (-SH) on the tryptophanases (TPases) from Escherichia coli B. and E. aurescens, Shigella alkalescens, and Proteus vulgaris and P. morganii. The coli group and the P. morganii apo TPases have 20 -SH groups per mole of enzyme, where as P. vulgaris apoTPase has 16. In coli group TPases, there are 16 -SH groups on the mole surface and they are all implicated in the activity and the enzyme-substrate bond. Proteus morganii TPase has 8 surface -SH groups, 4 of which are implicated in the activity; the remaining 12 -SH groups are located inside the mole and take part in the activity and the enzyme-substrate bond. Proteus vulgaris TPase has 4 surface -SH groups which are constructive of the enzyme structure, whereas the 12 remaining -SH groups are located inside the mole and take part in the activity and the enzyme-substrate bond. It is concluded that Proteus TPases are molecules which have inverted quaternary structure in comparison to those of the coli group. The studied TPases have four subunits, each of them is constituted of one polypeptidic chain having a molecular weight of 55,000.