A testing programme on Magnesian limestones from 30 sources has been used to demonstrate that the performance of these materials in concrete compares favourably with that of harder and more consistant materials such as Carboniferous limestones. Research has also shown a good correlation between durability and the size of pores within the aggregates. Both strength and frost resistance are discussed in relation to new specification guidance for aggregates drafted for inclusion in European Standards. This paper will help those responsible for the design and specification of concrete and for the planning of aggregate supply. It replaces IP2/91 which is now withdrawn. 6 pages.