This paper reports the chemical synthesis of three well-defined chains of deoxyribopolynucleotides bearing a 5′-phosphomonoester end group. The desired sequence of these protected oligonucleotides are related to a possible deoxyribonucleic acid sequence containing biological information for bovine insulin chain A. The new approach described in the present synthesis is that the chain length should be approximately doubled at each condensation step. Thus products and reactants differ substantially in molecular weight and can be separated rapidly by Sephadex gel filtration. An attractive feature of this separation technique is that the product peak emerges from the column before the peaking containing starting material and thus offers great advantages over ion-exchange DEAE-cellulose chromatography. The major objective for undertaking the synthesis and the general consideration for the selection of codons sequence relate to bovine insulin chain A is also presented. © 1969, American Chemical Society. All rights reserved.