Intracellular delivery of functional macromolecules using peptide transduction domains (PTDs) is an exciting technology with both experimental and therapeutic applications. Recent data indicate that PTD-mediated transduction occurs via fluid-phase macropinocytosis involving an intracellular pH drop to similar to 5. Nitrilotriacetic acid (NTA)-coordinated metals avidly bind hexa-histidine-tagged macromolecules, including peptides and proteins. Histidine's imidazole ring has a pK(a) of 6, making this an attractive target for the biological pH drop of PTD-mediated macropinocytotic delivery. The objective of this study was to develop a pH-sensitive PTD delivery peptide (NTA(3)-PTD). We demonstrate the in vitro function of this novel peptide by delivering fluorescently labeled peptides (1.6 kDa) and functional enzymes, beta-galactosidase (119 kDa) and Cre recombinase (37 kDa). Furthermore, the NTA(3)-PTD peptide was able to deliver functional Ore recombinase in an in vivo mouse model.