Previous studies showed that several lines of cells of the same type differed greatly in the amounts of interferon produced when stimulated by the same inducer (Cantell & Paucker, 1963; Lockart, 1965). In our laboratory, one line of L cells, designated LTS, produced relatively small amounts of interferon when induced with either Newcastle disease virus (NDV) or the complex of polyriboinosinic and polyribocytidylic acids (poly IC); another line of L cells, designated Lpa, produced significantly larger amounts of interferon when exposed to either of these inducers. It appears, therefore, that LTS cells have less interferon-producing potential than have Lpa cells. We showed recently that cells were sensitized by incubation with interferon so that induction by an otherwise non-inducing virus resulted in greater yields of interferon (Stewart, Gosser & Lockart, 1971). Also, since interferon was made earlier and in response to subinducing concentrations of poly IC, we proposed that pretreatment with interferon somehow altered the induction process.