Abstract: An improvement is described in the synthesis method for N.sup..alpha. -trityl-N.sup.G -trityl arginine within a process for preparing N.sup..alpha. -fluorenyl-methoxycarbonyl-N.sup.G -trityl-arginine from arginine, comprising:a) forming N.sup..alpha. -trityl-N.sup.G -trityl-arginineb) selectively detaching the trityl group from the .alpha.-NH.sub.2 andc) introducing the fluorenylmethoxycarbonyl group in its place.The improvement consists of preparing the N.sup..alpha. -trityl-N.sup.G -trityl-arginine by solubilizing the arginine in an aprotic organic solvent by tri-alkylsilylation both of the amino nitrogen and of the carboxyl group, followed by tritylation, with trityl chloride, of the .alpha.-amino nitrogen, and of the guanidino group after deprotonating this latter with a bicyclic guanidine. The new improved process can also lead to variable quantities of the arginine analogue di-tritylated at the guanidino group, namely N.sup..alpha. -fluorenylmethoxycarbonyl-N.sup.G -di-trityl-arginine.