Chemically and enzymatically synthesizeddiastereomeric 2′,5′- phosphorothioate dimer, trimer and tetramer cores and their 5′ - mono- and triphosphates demonstrate marked differences in their ability to bind to and activate RNase L from L929 cell extracts in radiobinding, core-cellulose and rRNA cleavage assays1,2 (Fig. 1). These are the first 2-5A cores that are able to bind to and activate Mase L. The enzymatically synthesized 2′,5′- phosphorothioate dimer and trimer 5′-triphosphates can also bind to and activate mase L1.