Core-shell polymers with highly cross-linked PMMA-cores and polystyrene shell have been synthesized in emulsion by the ＇＇grafting from＇＇ method using azo-modified seed latexes. These core-shell polymers were incorporated into a SBR elastomer matric using a kneader with subsequent crosslinking of the matrix. Stress-strain measurements showed a strong reinforcing effect of the core-shell polymers on the elastomer matrix. Dynamic mechanical measurements and TEM proved that the polymeric filler is homogeneously dispersed in the matrix.