The aim of cell therapy is to replace, repair, or enhance the function of damaged tissues or organs. Several factors complicate the development of cellular therapies. Of primary importance is protection of the implanted cells from the host's immune system. Cells are encapsulated in selectively semipermeable and biocompatible membranes that block entry of immune mediators but allow outward diffusion of active molecules produced by the cells. The immobilization of mesenchymal stem cells and monocytes, in micrometric (30-60 mu m) alginate-barium microcapsules based on atomization processes, has been achieved successfully. This size is necessary to the administration of microcapsules via injection (Hamilton syringe with a needle size of 100,mu m) and aerosol. Microencapsulated cells survive at least 2 weeks after preparation in vitro.