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The results presented above relate to several aspects of the 2-5A system. A simple but insensitive radiochemical assay fro 2-5A and its synthetase is described. Progress towards the molecular characterization of the synthetase suggested that it is composed of a single 56,000 dalton polypeptide chain that is synthesized in response to IF treatment. Degradation of 2-5A occurs at the same rapid rate in extracts of IF-treated and untreated chick cells. However, this breakdown can be inhibited by its end-product, 5'AMP, or by the activated synthetase which can further elongate 2-5A and thereby protect it from degradation. The direction of elongation is from the 5' to the 2' terminus. Molecules other than 2-5A can act as substrates fro 2'-adenylation by the activated synthetase. These include some dinucleoside monophosphates, ADP-ribose and NAD+, and methylene-bridged analogues of ATP. The methylene-bridged analogues of 2-5A that are synthesized in the latter case retain some of the biological activity of authentic 2-5A, indicating that cleavage of the 5'-terminal phosphate group(s) is not involved in the mechanism of nuclease activation by 2-5A.