A study of the influence of employing ultrahigh molecular weight polyethylene (UHMWPE) on the toughness of CaCO3/high-density polyethylene (HDPE) composites was carried out. Binary and ternary HDPE-based composites with calcium carbonate in the range of 0-40% and UHMWPE in the range of 0-50% were produced by twin-screw extrusion followed by compression molding. From tensile and impact tests, it was found that increasing calcium carbonate content increased tensile modulus, but decreased tensile strength, strain at break, and impact resistance. The addition of UHMWPE helped to increase the strain at break and impact resistance of composites moderately without decreasing modulus or strength. The degree of toughening was found to increase with increasing UHMWPE content, but to decrease as the filler volume fraction was increased. (C) 2000 John Wiley & Sons, Inc.