In a method for controlling the growth of microbes grown in a medium to high cell-densities by the fed-batch technology, a two-phase system, having a liquid phase (cultivation medium) and a solid or gel phase, the solid or gel phase provides a source of substrate-delivering polymer which is turned by an enzyme in a controlled way into a growth-limiting substrate or pH adjusting agent. Also described is a method for restricting the synthesis of growth-limiting metabolites and preventing oxygen-depletion in microbial cell cultures.