This work investigates the influences of impurities on the durability and the reliability of the mechanical properties of recycled glass fiber reinforced polyamide 66. A critical size-concentration zone could be determined based on changes in tensile properties of samples containing untreated glass beads of different size and concentrations, which were used as simulated impurities in in-plant reprocessed material. Characterization of samples based on recyclate from post-use radiator end-caps, which contained residues of EPDM rubber, showed that the tensile strength was almost unaffected by rubber particles up to an equivalent diameter, d, of similar to 700 mu m However, a significant strength reduction commenced for rubber particles larger than 1200-1300 mu m, and the strength showed a correlation with d(-1/2). Furthermore, large simulated impurities (425-590 mu m) in in-plant material and rubber residues in post-use recyclate showed a negative effect on the mechanical reliability.