A. Introduction Friend erythroleukemia cells (FLC) are proerythroblasts chronically infected with the Friend virus complex (FLV, composed by LLV and SFFV). They undergo erythroid differentiation accompanied by synthesis of heme and hemoglobin (Hb), accumulation of globin mRNA and erythrocyte-specific membrane changes upon treatment with DMSO and other polar solvents [6] . Data collected in the past 2-3 years have convincingly shown widespread pleiotropic effects of interferon (IF). As reviewed

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... lsewhere [I], inhibitory as well as stimulatory effects of IF on various types of cells have been reported. As for the antiviral activity of IF, it has been shown that in cell chronically infected wi RNA tumor viruses, IF treatment suppresses extracellular virus production, but does not abolish intracellular virus antigens expression [4]. We have been studying the action of DMSO and IF upon growth and differentiation of FLC, on one hand, and the effect of IF on the FLV genome expression, on the other hand. B. Control of FLC Differentiation by Interferon In previous Papers we have described in detail the effects exerted by high doses of IF on FLC growth, cell cycle and erythroid differentiation [9, 11, 12] . FLC growth is inhibited, in a dose-dependent fashion, by the administration of IF doses higher than 500 U/ml. Ce11 cycle parameters also change with respect to GI and Gs phases, that are almost doubled in length. Removal of IF results in a prompt resumption of growth potential and normal cycle parameters. Similar data were obtained when DMSO-stimulated FLC were treated with IF. Under conditions allowing at least 2-3 cell doublings in the presence of DMSO, Hb synthesis was reduced by 90%, but globin mRNA only by 30%, in FLC treated with DMSO + IF. It is apparent that the observed transcriptional effect of IF, although a novel one for a "cellular" mRNA, cannot fully explain the magnitude of the translational inhibition. In addition, these globin mRNAs (from DMSO-and from DMSO + IF-treated FLC) are indistinguishable from one another for a) base sequence, as deter-