This study shows, for the first time, the advantages of combining two transition-metal complexes as selective proteolytic reagents. In this procedure, cis-[Pt(en)(H2O)(2)](2+) is followed by [Pd(H2O)(4)](2+). In the peptide AcAla-Lys-Tyr-Gly-Gly-Met-Ala-Ala-Arg-Ala, the Pt(II) reagent cleaves the Met6-Ala7 peptide bond, whereas the Pd(II) reagent cleaves the Gly4-Gly5 bond. In the peptide AcVal-Lys-Gly-Gly-His-Ala-Lys-Tyr-Gly-Gly-Met-Ala-Ala-Arg-Ala, the Pt-(II) reagent cleaves the Met11-Ala12 peptide bond, whereas the Pd(II) reagent cleaves the Gly3-Gly4 bond. All cleavage reactions are regioselective and complete at pH 2.0 and 60 degreesC. Each metal ion binds to an anchoring side chain and then, as a Lewis acid, activates a proximal peptide bond toward hydrolysis by the solvent water. The selectivity in cleavage is a consequence of the selectivity in this initial anchoring. Both Pt(II) and Pd(II) reagents bind to the methionine side chain, whereas only the Pd(II) reagent binds to the histidine side chain under the reaction conditions. Consequently, only methionine residues direct the cleavage by the Pt(II) reagent, whereas both methionine and histidine residues direct the cleavage by the Pd(II) reagent. The Pt(II) reagent cleaves the first bond downstream from the anchor, i.e., the Met-Z bond. The Pd(II) reagent cleaves the second bond upstream from the anchor, i.e., the X-Y bond in the X-Y-Met-Z and in the X-Y-His-Z segments. The diethylenetriamine complex [Pt(dien)(H2O)](2+) cannot promote cleavage. Its prior binding to the Met11 residue in the second peptide prevents the Pd(II) reagents from binding to Met11 and cleaving the Gly9-Gly10 bond and directs the cleavage by the Pd(II) reagent exclusively at the Gly3-Gly4 bond. Our new method was tested on equine myoglobin, which contains 2 methionine residues and 11 histidine residues. The complete methionine-directed cleavage of the Met55-Lys56 and Met131-Thr132 bonds by the Pt(II) reagent produced three fragments, suitable for various biochemical applications because they are relatively long and contain amino and carboxylic terminal groups. The deliberately incomplete histidine-directed cleavage of the long fragments 1(...)55 and 56(...)131 at many sites by the Pd(II) reagent produced numerous short fragments, suitable for protein identification by mass spectrometry. The ability of combined Pt(II) and Pd(II) complexes to cleave proteins with explicable and adjustable selectivity and with good yields bodes well for their greater use in biochemical and bioanalytical practice.