Multicomponent, in situ foaming system for the preparation of interpenetrating polymeric networks (IPN) of foamed polyurethane and at least one further polymer for in situ construction purposes with a polyisocyanate component (A) and a polyol component (B) for forming the polyurethane, and further components (C) and (D) for forming the further polymer, components (A) and (B) being present in a reaction-inhibiting, separate form, characterized in that the components (A), (B), (C) and (D) are present in the form of one or two mixtures, in which the components (A), (B), (C) and/or (D) are contained separately in a micro-encapsulated form in order to inhibit reaction so that the components polymerize with formation of the interpenetrating polymeric network only when the components are brought into contact with one another after destruction or opening of the microcapsules.