We utilized alginate sodium (ALG) and chitosan (CHI) as wall components to construct a natural and biodegradable polyelectrolyte micro-shell by the electrostatic self-assembly technique. The resultant ALG-CHI shells were found to maintain intact spherical shape in different poly (styrenesulfonate sodium) (PSS) bulk concentrations (maximal detection concentration 20.0 wt%). Compared with the shells composed of PSS and poly(allylamine hydrochloride) (PAH) that collapse or deform at a 4 wt% or more than 4 wt% PSS concentration, the ALG-CHI shells display higher stability. Meanwhile, the permeability of the ALG-CHI shells prior to and after incubation in PSS solutions was directly traced by the fluorescence recovery after photobleaching (FRAP). The composition of the ALG-CHI shells themselves is thought to be responsible for the differences in the stability. (c) 2006 Elsevier Ltd. All rights reserved.