Polish PP (Malen P J-400) filled with 40% of (a) 98% pure CaCO3, (b) 32%CaO-19.5% MgO (dolomite P-10), or (c) 30% of talc + stabilizer and lubricant, fed n various ways into a 2T4OWx45D extruder, was examined for MFR(230; 2.16), tensile strength, Izod notched impact strength, and filter dispersion degree. Mechanical properties were closely related to the degree of filling and amount and type of auxiliaries. As filler content was increased, the tensile strength, elasticity module, and elongation fell, whereas rigidity and notch impact strength rose. Talc-based composites were mechanically best, whiting-based ones were worst. Temperature in the side-feeding zone (filler), degree of filling of grooves in the intensive melt mixing zone, and the geometry of screws in this zone, were important factors. The PP filled with a powdered mineral at temperatures exceeding the highest m.p. (as determined by DSC) had the best physical properties.