An efficient transition metal catalyzed procedure for the reductive alkylation of proteins has been developed. Imines formed from the condensation of aldehydes (1 mM) with lysine residues and the N-terminus can be reduced efficiently by a [Cp*Ir(4,4'-dimethoxy-2,2'-bipyridine)(H2O)]SO4 catalyst in the presence of formate ions. The reaction proceeds readily at pH 7.4 in aqueous phosphate buffer at temperatures ranging from 22 to 37 degrees C, and reaches high levels of conversion for a number of aromatic aldehydes. UV experiments have confirmed that the catalyst does not bind to protein substrates. The utility of the reaction has been demonstrated through an efficient two-step procedure for the attachment of unfunctionalized poly(ethylene glycol) to protein targets.