An inhibitor of interferon action has been identified which is present in IFN-γ preparations. The inhibitor is produced following rising interferon concentrations in mitogen stimulated mouse spleen cell cultures. Indications are that the inhibitor is produced in response to the production of interferon, and may therefore be a feedback control mechanism for the interferon system. The action of the inhibitor is independent of both interferon type and concentration, and seems to act by preventing the establishment of the interferon-induced antiviral state at some point following the interaction of the interferon molecule with the cell membrane. The inhibitor has an apparent molecular weight of 8,000 to 10,000 daltons and is stable to treatment with low pH, heat, and trypsin. It is proposed that the inhibitor has the specific role in vivo of controlling interferon-mediated activities.