When L929 cells were treated with samples collected from cells or mice producing interferon, almost complete refractoriness to subsequent interferon induction was observed. The relationship of this refractoriness-inducing product (RIP) to the antiviral activity of the samples was determined. Using interferon in the pretreatment sample as a measure of RIP concentration, a semilog plot of the pretreatment interferon titer and interferon subsequently produced, resulted in an approximately linear relationship between 10 and 100 units of interferon in the pretreatment sample. This relationship was largely independent of multiplicity of Newcastle disease virus (NDV) used to produce interferon in the treated monolayers. A 2- to 6-fold temporal differentiation in the refractoriness-inducing activity and the antiviral activities was obtained both in vitro and in vivo. Supernatants collected 48 hr after NDV infection of L929 cells had 2- to 4-fold more refractoriness-inducing activity than those collected earlier. In serum of ICR mice given poly rIn:rCn-poly-D-lysine, a quantitatively similar temporal increase in refractoriness-inducing activity was observed. Treatment of L929 cells with cycloheximide resulted in a 4-fold increase in interferon production. At equivalent interferon concentrations, supernatants from the cycloheximide-treated monolayers inhibited interferon production 4-fold less than did comparable supernatants from non-cycloheximide-treated monolayers. Soluble cell extracts inhibited interferon production to a greater degree than did extracellular preparations. The protein nature of the refractoriness-inducing activity was suggested by its trypsin sensitivity. These findings suggest that the refractoriness-inducing and antiviral activities of interferon preparations result from different proteins.