Magnesium alloys are one of the lightest of all the structural materials. Because of their excellent physical and mechanical properties the alloys have been used more and more often in various branches of industry. They are cast mainly (over 90%) on cold and hot chamber die casting machines. One of the byproducts of casting processes is process scrap which amounts to about 40 to 60% of the total weight of a casting. The process scrap incorporates all the elements of gating systems and fault castings. Proper management of the process scrap is one of the necessities in term of economic and environmental aspects. Most foundries use the process scrap, which involves adding it to a melting furnace, in a haphazard way, without any control of its content in the melt. It can lead to many disadvantageous effects, e.g. the formation of a hard buildup at the bottom of the crucible, which in time makes casting impossible due to the loss of the alloy rheological properties. The research was undertaken to determine the effect of an addition of the process scrap on the mechanical properties of AZ91 and AM50 alloys. It has been ascertained that the addition of a specific amount of process scrap to the melt increases the mechanical properties of the elements cast from AZ91 and AM50 alloys. The increase in the mechanical properties is caused mainly by compounds which can work as nuclei of crystallization and are introduced into the scrap from lubricants and anti-adhesive agents. Furthermore carbon, which was detected in the process scrap by means of SEM examination, is a potent grain modifier in Mg alloys [1-3]. The optimal addition of the process scrap to the melt was determined based on the statistical analysis of the results of studies of the effect of different process scrap additions on the mean grain size and mechanical properties of the cast parts.