Changes in the behavior of alginates, both non-gelled and gelled with calcium ions, in microcapsules after implantation in vivo were investigated using alginates enclosed in regenerated cellulose membrane. The mechanical properties of the alginates after soaking in simulated body fluid (SBF) and implantation in the peritoneal cavities of animals were determined. A stress-compression study on specimens soaked in SBF showed that the alginates in the microcapsules would have almost the same mechanical properties after several days of implantation independent of whether the enclosed alginate was non-gelled or gelled with calcium ions before implantation. The stresses at a particular compression point decreased with increasing incubation period for 3-6 days for the alginates gelled with calcium ions before soaking in SBR In contrast, these values increased over the same period for the non-gelled alginates before soaking in SBR In agreement with the results of the in vitro study, no significant differences were observed between the mechanical properties of the non-gelled and gelled alginates retrieved from the peritoneal cavities of DDY mice after 1 week of implantation. (C) 2004 Elsevier B.V. All rights reserved.