Protein arginine N-methyltransferase (PRMT1) is one of the proteins that bind to the intracytoplasmatic domain of the IFNAR-1 chain of the type I interferon (IFN) receptor system. The attachment is specific and is not seen with PRMT2, another member of this protein family. Antisense PRMT1 cDNA constructs expressed under the early cytomegalovirus (CMV) promoter were transfected into HeLa cells, and stable transformants were selected. Antibodies to PRMT1 were used to identify clones with reduced PRMT1 expression. In such clones, IFN-beta inhibited three to five times less the replication of vesicular stomatitis virus (VSV) than in the original HeLa cells. The antiproliferative effect of IFN-beta was also reduced up to fivefold in the clones with low PRMT1 expression. No difference was seen when IFN-gamma was used alone to inhibit cell growth. The protein methylating enzyme, bound to IFNAR-1, appears to regulate positively the biologic activity of type I IFN.