Here we extend the investigation described in the preceding companion communication. To rectify its notch brittleness, high-density polyethylene (HDPE) was modified by rigid particulate fillers consisting of three different sizes of CaCO3 particles of 3.50, 0.70 and 0.44 mu m weight average diameter in various volume fractions. Mechanical properties including notched Izod impact energy of the extrusion-blended/ injection-moulded samples were examined as a function of filler particle size and filler volume fraction. In exactly the same manner as exhibited by the rubber particle-modified compositions described in the previous communication, the toughness of the CaCO3-filled materials increased dramatically when the mean interparticle ligament thickness of the matrix polyethylene dropped to values below 0.6 mu m. The stiff fillers used in this study provided the unusual additional benefit of substantially increasing the Young＇s modulus of the compounds while also dramatically improving their impact energy.