Abstract: An improved process is provided for producing cigarette filters comprised of a substantially open-cell foamed thermoplastic polypropylene resin wherein a mixture comprising a thermoplastic polypropylene resin, a volatile blowing agent and a nucleating agent is extruded through a die to produce a strand of foamed resin with the strand then being cut into individual filter elements. The improvement comprises employing as a primary nucleating agent from 0.1 to 40 percent by weight, based on the weight of the resin, of a particulate compound of a metal selected from Groups IIA, IIIA or IVA of the Mendeleef periodic table of elements together with from 0.01 to 5.0 percent by weight, based on the weight of the primary nucleating agent, of a synergist consisting of an organic compound having a hydrophobic group and a hydrophilic group. A particularly effective combination is the use of calcium carbonate as the primary nucleating agent and potassium benzoate as the synergist compound.