We have found a novel way of protecting the hydroxyl function in hydroxy- L-amino acids, making use of the t-butoxy group, which is readily split by acids, but not by bases. This group is introduced by acid-catalysed addition of isobutene to N-acylated hydroxy-L-amino acids. The blocking group is easily removed at or below room temperature with trifluoroacetic acid without racemization and without breaking peptide bonds 1,s. ... The hydroxy-L-amino acids serine, threonine, ...