A novel method for removal of protective group(s) from an amino acid or peptide having thiol group(s) protected with p-methoxybenzyl, 1-adamantyl or t-butyl group by treating the protected amino acid or peptide with the mercuric salt of acetic acid or halogenoacetic acid, the removal being effected selectively and very smoothly in a good yield under extremely mild conditions without giving any undesirable influence upon other functional or protective group(s).