The human interferon (hIFN alpha 1) gene contains 11 arginine (Arg) codons AGG or AGA, which are extremely rare for bacteria, four of which are organized in tandems. The two AGG tandems (corresponding to Arg12 Arg13 and Arg163 Arg164) are known to inhibit the translation of hIFN alpha 1 mRNA and therefore they are considered to be responsible for the poor expression of hIFN alpha 1 gene in bacterial cells. To study the effect of these two tandems on the expression of hIFN alpha 1 in E. coli, four new gene variants were designed to contain preferential Arg codons (CGT) substituted for the rare AGG codons in either the first, the second or both AGG tandems. We found that, whereas the yield of hIFN alpha 1 protein per cell remained unchanged, the level of hIFN alpha 1 mRNA decreased gradually (by a factor of two) with the consecutive substitution of the first, second and both AGG tandems. These results indicated, first, that the AGG clusters might have a stabilizing effect on the mRNA, and second, that mRNAs devoid of such clusters were translated at a higher rate in vivo. The protein products of the four genes (having the same amino acid sequence) showed different specific antiviral activity. The most active was the product of gene hIFN alpha 1(c) in which the second AGG tandem (corresponding to Arg163, Arg164) was preserved while the least active was the protein of gene hIFN alpha 1(d) (devoid of both AGG clusters). The role of the AGG tandems in folding of the gene product is discussed.