The chemical modification of proteins is of great importance in chemical biology, biotechnology, and for the production of modified biopharmaceuticals, as it enables introduction of fluorophores, biotin, half-life extending moieties, and more. We have developed two methods that use poly-His sequences to direct the highly selective acylation of proteins, either at the N-terminus or at a specific Lys residue. For the former, we used an N-terminal Gly-His(6) segment (Gly-His tag) that directed acylation of the N-terminal N-alpha-amine with 4-methoxyphenyl esters, resulting in stable conjugates. Next, we developed the peptide sequences His(n)-Lys-His(m) (Lys-His tags) that direct the acylation of the designated Lys N-epsilon-amine under mild conditions and with high selectivity over native Lys residues. Both the Gly-His and Lys-His tags maintain the capacity for immobilized metal ion affinity chromatography. We have demonstrated the robustness of these methods by attaching different moieties such as azides, fluorophores, and biotin to different proteins, including antibodies.