Abstract: The invention relates to a novel process for the production of (2'-5')A(pA).sub.n pA chains wherein n is an integer of from 1 to 6, which comprises polymerizing adenosine (2'-3') monophosphate to obtain a mixture of 2'-5'- and 3'-5'-polyadenylic acid, treating the mixture with ribonuclease P.sub.1 to cleave selectively the 3'-5'-bonds so as to retain the 2'-5'-polymeric moieties with a terminal 5'-phosphate moiety and removing the terminal phosphate groups by means of alkaline phosphatase. The mixture may be purified by separation on an anionic ion exchanger and collecting the individual fractions. The invention further relates to pharmaceutical compositions for inhibiting the immune response of mammals which comprise as active ingredient a (2'-5')oligo-isoadenylate defined above. The preferred compositions are those with a preponderance of the trimer.