The determination of 2'-5'-oligoadenylate synthetase in peripheral blood mononuclear cells is used as a biological response parameter during therapy with interferon and in the diagnosis of diseases related to the interferon system. In this communication, some general aspects concerning the preparation of 2'-5'-oligoadenylate synthetase from peripheral blood mononuclear cells and the incubation conditions of the 2'-5'-oligoadenylate synthetase reaction are reported. Four analytical procedures for the determination of the products formed during the 2'-5'-oligoadenylate synthetase reaction were comparatively investigated and the advantages and limitations of the assays are discussed. As an example of possible clinical application, the levels of 2'-5'-oligoadenylate synthetase were determined in the mononuclear cell fraction of peripheral blood from healthy persons as well as from children with chronic myelogenous leukaemia.