The Cu(i)-catalyzed cycloaddition of terminal azides and alkynes (click chemistry) represents a highly specific reaction for the functionalization of biomolecules with chemical moieties such as dyes or polymer matrices. In this study we evaluate the use of bicinchoninic acid (BCA) as a ligand for Cu(i) under physiological reaction conditions. We demonstrate that the BCA-Cu(i)-complex represents an efficient catalyst for the conjugation of fluorophores or biotin to alkyne- or azide-functionalized proteins resulting in increased or at least equal reaction yields compared to commonly used catalysts like Cu(i) in complex with TBTA (tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl] amine) or BPAA (bathophenanthroline disulfonic acid). The stabilization of Cu(i) with BCA represents a new strategy for achieving highly efficient bioconjugation reactions under physiological conditions in many application fields. © 2012 The Royal Society of Chemistry.