A simple methodology for single-site conjugation of macrocycles and metallomacrocycles with biotin is presented. This method has been used to conjugate a redox-active macrocyclic complex and embed it into avidin. The resulting biotin-metal complex-avidin adduct exhibits peroxidase activity as shown by the oxidation of ABTS and pyrogallol with H2O2. © 2009 Springer Science+Business Media B.V.