The interferon-inducible enzyme, 2'-5'-oligoadenylate synthetase, was estimated in healthy donors and in patients before and after subcutaneous application of recombinant interferon beta and gamma. Tests were carried out with lysates of peripheral blood mononuclear cells, using an established radioenzymatic assay, and in serum samples, using a new radioimmunoassay. Both test systems substantially yielded the same results: after a single injection of interferon beta (1-5 x 10(6) IU), 2'-5'-oligoadenylate synthetase increased in a dose-dependent manner reaching maximal catalytic concentrations in most patients after 24-48 hours (leukocytes) and 48-72 hours (serum). In contrast, interferon gamma (2-4 x 10(6) IU) caused only a small induction of 2'-5'-oligoadenylate synthetase. However, daily application of interferon gamma for 7 days led to a distinct time-dependent increase of 2'-5'-oligoadenylate synthetase activity concentration during this observation period. Characteristically, even during daily application, the 2'-5'-oligoadenylate synthetase activity concentration dropped just 48-72 hours after the first injection of interferon beta. The determination of 2'-5'-oligoadenylate synthetase proved to be useful for optimizing and monitoring subcutaneous therapy with interferon. The new radioimmunoassay which allows the determination of this enzyme in serum is superior to other methods used in the past.