The hydrolysis of TAME by bovine and human thrombin was inhibited about 40% by 0.1 M NaCl but not by even higher concentrations of KCl. Probably Na+, and not an impurity in the NaCl used, produced the inhibition because NaCl from a second source gave the same results, and in addition, rates of hydrolysis when using a buffer made from the mono- and dipotassium phosphates were double those found when using a buffer made from the corresponding sodium phosphates. No evidence was found that Na+ inhibited, not thrombin, but a contaminant in the 4 thrombin preparations tested. Increasing concentrations of glycerol in the tests produced increasing inhibition of hydrolysis, but the percentage inhibition due to Na+ remained constant. Incubating thrombin at 37° and pH 3.4 gradually destroyed its ability to hydrolyze TAME, but at all times tested the percentage inhibition due to Na+ remained constant. SBTI did not inhibit the rates, and the percentage inhibition by Na+ was the same in the presence and in the absence of SBTI. For these reasons it was concluded that thrombin itself is inhibited by Na+.