The production of human lymphoblastoid interferon messenger RNA synthesis was characterized in Namalva cells after induction with Newcastle disease virus. The level of interferon mRNA increased 50- to 100-fold within 8 hours of induction. The amount remained constant for an additional 6 hours but began to decrease by 17 hours postinduction. Under the latter conditions the entire cellular complement of interferon mRNA was found free in the cytoplasm. In contrast, 50% of the interferon mRNA was polysome-associated earlier, during active synthesis of the antiviral protein. At both times interferon mRNA was fully translatable in oocytes of Xenopus laevis. Since interferon protein is virtually fully induced 11 hours after the initiation of virus treatment, the shut-off of interferon synthesis cannot be attributed to decay of its mRNA. These results show that control of interferon synthesis in Namalva cells is a function of mRNA translatability, not of mRNA stability. The data explain, in part, why superinduction of interferon mRNA has never been observed in these cells. Immune interferon mRNA was isolated from human lymphocytes stimulated with staphylococcal enterotoxin A. The yield is approximately 200-fold lower than the yield of lymphoblastoid interferon mRNA from Namalva cells.