Poly(ethylene imine) (PEI), a highly cationic polymer, is being used for deoxyribonucleic acid (DNA) complexation and delivery into cells. To enhance the cellular uptake of polymer/DNA complexes, arginine-glycine-aspartic acid (RGD) peptides have been conjugated to PEI with N-succinimidyl 3-(2-pyridyldithio)propionate (SPDP). This coupling scheme creates a disulfide-linked conjugate, the stability of which in the presence of thiols is uncertain. We have investigated the conjugation of an RGD peptide, glycine-arginine-glycine-aspartic acid-serine-proline-cysteine (GRGDSPC), to PEI with SPDP and subsequently assessed the stability of the conjugates in the presence of two thiol compounds, mercaptoethanol and cysteine. SPDP effectively controls the extent of GRGDSPC substitution on PEI. The conjugates, however, are readily cleaved in the presence of the thiols; the cleavage is rapid (ϳ50% cleavage in 2-4 h) and inversely related to the degree of peptide substitution on the polymers. The peptide coupling is stable in the absence of thiols, and its cleavage is strongly dependent on the pH of the medium but not on the ionic strength of the medium. We conclude that RGD peptides coupled to PEI are labile in the presence of physiological concentrations of thiols, and this should be taken into account when such polymer-peptide conjugates are used for DNA delivery.