A reinforcement effect of polymeric fillers is demonstrated which is not based on physical interactions between filler and rubber matrix (e.g. as for carbon black), but solely on chemical bonds formed by grafting onto the rubber matrix during vulcanisation. SBR-latices mixed azo containing. crosslinked PMMA-latices were used for this purpose. These ＇＇reactive fillers＇＇ yield a much higher tensile strength and improved elongation at break compared to non-reactive polymeric fillers. The resulting tilled and reinforced SBR samples are optically clear.