Polyethylene (PE) and Polypropylene (PP) are regarded as the most popular plastics of our time. While conquering large fields of the plastic market, particularly in the field of long-term structural applications, such as pipe systems, the increasing production of those polymers generates an aspiration for recycling that enormous amount of consequential waste. This, however, has raised serious discussions throughout the globe evoking recommendations for or against the use of post-consumer or post-industrial recycled plastics (known as non-virgin materials). Implementing a non-virgin material in a pipe system usually means via blending approaches or in form of various sandwich structures. Yet, the influences of non-virgins on lifetime relevant properties of virgin materials have not been investigated properly. Categorising the influence of non-virgin on virgin plastics into three groups – •influence of virgin polymeric impurities without any associated non-polymeric contaminations (e.g. amount of PP in PE), •influence of polymer additives and inorganic impurities (e.g. amount of compatibilizer/stabilizer, fillers or metallic dirtying) and •influence of changes of molecular mass and molecular mass distribution through reprocessing steps during recycling or mixing of grades for different applications (e.g. short-chained blow-molding PE in PE100 pipe grades with high molecular mass) – the principal objective of this work is attributed to the examination of the severity of virgin polymeric impurities incorporated by virgin polymers regarding their resistances to crack initiation and Slow Crack Growth (SCG). A comprehensive study was achieved by observing the alteration in lifetime relevant material characteristics of a virgin PE pipe grade (PE80) with successively increasing amounts of a virgin PP block copolymer (PP-B) (so-called “scientific grades”). While calculated SCG resistances based on Strain Hardening tests and expressed by the SH modulus give unrealistic results, the predicted SCG performances gained from Cyclic Cracked Round Bar (CRB) tests, which are reflected by the total failure cycle numbers Nf, emphasize a significant degradation of SCG resistances, provoked by the implementation of contaminating polyolefins. Already 5 % mixing content evokes a dramatic decrease of SCG resistances up to 40 and 70 % for PE80 and PP-B, respectively. A similar reduction of crack initiation times Nini is also observable. In conclusion, results of this study confirm the validity of the CRB test as a tool to rank modern PE and PP grades by their real SCG resistance. On the other hand, the results demonstrate the high negative impact of already small amount of polymeric impurity, which leads to a significant reduction of the SCG resistance. As SCG resistance is a major material property for long-term application, this effect must be carefully considered in all discussions about the use of non-virgin polymers for pipe applications.