- The effect of interferon (IFN) on virus release and on assembly of intracellular virions in 3T3/NIH cells chronically infected with Moloney murine leukemia virus was studied by short labeling with3H-leucine (viral proteins),3H-glucoseamine (viral glycoproteins) and3H-uridine (viral RNA). With all of these labels, IFN pretreatment was found to strongly inhibit extracellular virus release. No difference was found between the extent of labeling of viral proteins and glycoproteins of intracellular virions. Incorporation of3H-uridine into intracellular virions was strongly reduced by the IFN pretreatment. Since it is rather unlikely that encapsidated RNA is significantly degraded during the relatively short time of label incorporation, this finding suggests that IFN interfers with packaging of viral RNA. The effect of IFN on virus release and on3H-uridine labeling of intracellular virions were found to develop with the same kinetics, though the maximal inhibition of virus release was stronger.