Cystamine, when employed as a cross-linking agent, leads to poly(amidoamine) networks, which on reaction with 2,2-dithiodipyridine turn into linear poly(amidoamine)s with side dithiopyridyl groups that easily undergo exchange reactions with reduced L-glutathione, a model thiol-containing biologically active peptide. The resultant products represent the first examples of soluble poly(amidoamine)-peptide conjugates in which the peptide moieties are linked to the polymer chain by S-S bonds stable in blood, but cleavable inside cells.