The interaction between poly-L-lysine (pLys) and oppositely charged poly(acrylic acid) (pAA) microgels (empty set similar to 80-120 mu m) was studied by micromanipulator-assisted light microscopy and confocal laser scanning microscopy. The aim of this study was to investigate effects of peptide size, pH, and salt concentration on binding, transport, and distribution of pLys in pAA microgel particles and thereby also to outline the details of the gel deswelling in response to pLys binding and incorporation. Both peptide distribution and gel deswelling kinetics were found to be strongly influenced by the pLys molecular weight, originating partly from limited entry of large peptides into the gel particle core. Also pH was shown to influence both deswelling and pLys incorporation kinetics, with a decreased deswelling rate observed with increasing pH. These effects are determined by a complex interplay between the pH-dependence of both pLys and the gel network, also influencing volume transitions of the latter. Finally, salt concentration was shown to have a significant effect on both gel deswelling rate and pLys transport, with an increased electrolyte concentration resulting in decreased deswelling rate but also in an increased peptide transport rate within the microgel particles. (C) 2008 Elsevier Inc. All rights reserved.