AbstractThe efficiency of protein chemical modification on tyrosine residues with N‐methylluminol derivatives was drastically improved by using horseradish peroxidase (HRP). In the previous method, based on the use of hemin and H2O2, oxidative side reactions such as cysteine oxidation were problematic for functionalization of proteins selectively on tyrosine residues. Oxidative activation of N‐methylluminol derivatives with a minimum amount of H2O2 prevented the occurrence of oxidative side reactions under HRP‐catalyzed conditions. As probes for HRP‐catalyzed protein modification, N‐methylluminol derivatives showed much higher efficiency than tyramide without inducing oligomerization of probe molecules. Tyrosine modification also proceeded in the presence of β‐nicotinamide adenine dinucleotide (NADH, H2O2‐free conditions).