Résumé : Metabolically stable analogues of (2′‐5′)oligo(adenylate), (2′‐5′)(A)n. might constitute a new class of antiviral agents as they mimic some of the effects of interferons. 2′‐O‐phosphoglyceryl derivatives of (2′‐5′)(A)n oligomers, (2′‐5′)(A)n‐PGro have been synthesized by chemical modification of their terminal ribose residue. Such analogues are resistant to degradation by phosphodiesterases but remain sensitive to phosphatase activity, at least in cell‐free extracts. In line with its increased stability, (2′‐5′)(A)n‐PGro has a powerful antiviral activity against an RNA virus when microinjected with micropipettes into the cytoplasm of intact cells. This antiviral activity remains transient however, possibly as a consequence of degradation in intact cells. Since (2′‐5′)(A)n and its derivatives do not easily cross cell membranes, their possible use in antiviral chemotherapy is tightly linked with the development of vectors suitable for their administration in vivo. Copyright © 1984, Wiley Blackwell. All rights reserved