NG-mesitylene-2-sulfonylarginine was prepared. This protecting group was found to be quantitatively cleaved by hydrogen fluoride, methanesulfonic acid or trifluoromethanesulfonic acid and partially by hydrogen bromide in acetic acid. As a side reaction, partial mesitylenesulfonylation of the phenolic group of tyrosine was noted. However this side reaction could be suppressed by cation scavengers, anisole-thioanisole-cresol (o) (1 : 1 : 1), in less than 4%.