Brittle thermoplastics are hardened and embrittled by mineral fillers and softened and (sometimes) toughened by elastomers. We investigated the possibility of combining these effects favorably in filled blends of a thermoplastic, polystyrene (PS); an elastomer, polybutadiene (BR); and a filler, chalk. The success had to be measured in comparison to commercial high-impact polystyrene (HIPS) which is produced by in situ polymerization. At low concentrations of BR, simple blends of PS/BR are tougher than PS itself, but not considerably. This could be improved by adding chalk. The blends PS/BR/chalk feature a core-shell domain morphology, with BR enveloping chalk aggregates on the micrometer scale. At BR contents of less than 10 vol %, the stress-strain behavior of the filled blends PS/BR/chalk compares well to that of HIPS. The blends exhibit multicrazing with characteristic patterns and can be bent easily without breaking. At higher BR contents, however, the blends go back to brittle failure.