Interferons are regulatory secretory proteins that have a variety of biological activities, including antiproliferative actions against tumor cell lines and tumors in vivo. Among the several interferon-inducible genes, modulation of 2'-5'-oligoadenylate synthetase expression has been the focus of considerable attention because of its implication in growth regulation. In the studies presented here, we report growth inhibition of A431 cells by gamma-interferon and provide new evidence to support the possible involvement of 2'-5'-oligoadenylate synthetase. We observed that the antiproliferative effect of gamma-interferon on cultures of A431 cells was accompanied by prolongation of G0-G1 phase and profound morphological changes, which were progressively prominent from day 4 onward. The levels of 2'-5'-oligoadenylate synthetase enzyme activity and mRNA were significantly elevated following gamma-interferon treatment and correlated well with the kinetics of growth-inhibitory effects. There were differential induction and expression of different isoenzyme forms of 2'-5'-oligoadenylate synthetase: the Mr 100,000 and Mr 40,000 proteins were induced maximally to varying degree by day 4, with subsequent decline; in contrast, the Mr 67,000 protein was induced late and remained high up to day 9 after gamma-interferon treatment. In accord with these observations, fractionation of enzymatic activity also revealed differences in the enzyme activities of 2'-5'-oligoadenylate synthetase forms from control and treated cells. Therefore, these observations demonstrate the existence of possible differential regulation of 2'-5'-oligoadenylate synthetase isoenzymes and strengthen the concept that growth inhibition of cells may be mediated via induction of 2'-5'-oligoadenylate synthetase.