Certain preparations of crude chick interferon had earlier been found to inhibit interferon production as well as virus growth. This inhibition is now shown to be due, not to interferon, but to an inhibitor for which the name “blocker” is proposed. Blocker was found in the allantoic fluid of eggs infected with fowl plague, Newcastle disease virus and B/England/939/59 influenza virus, but not so far in four other strains of influenza virus. Blocker resembles interferon in a number of properties: heat stability, nondialyzability, lack of sedimentation at 105,000 g for 2 hours, stability at pH 2, absence of neutralization by viral antiserum and presence in greater amount in the infected allantoic fluid than in the chorioallantoic membrane. However, blocker differs from interferon in two main ways. Firstly, on partial purification of interferon, blocker becomes gradually separated from interferon. Secondly, on treatment with trypsin there was either no change or a small rise in the activity of blocker, and on treatment with pepsin there was a clear rise. Blocker may be considered as a repressor of the synthesis of interferon.