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L-adenosine was prepared starting from L-arabinose according to Holý and Sorm (Collect. Czech. Chem. Commun. 34 (1969), 3383-3401) via benzyl-β-L-arabinopyranoside and transformation into 2-O-tosyl-5-O-trityl-L-arabinose. For the synthesis of L-uridine, at first the 2,2′-O-anhydro-L-uridine was depicted from L-arabinose according to Holý (Collect. Czech. Chem. Commun. 37 (1972), 4072-4087). Subsequently, it was benzoylated to the 3′,5′-di-O-benzoyl-derivative according to Holý (Collect. Czech. Chem. Commun. 38 (1973), 423-427) and after a reaction with boron trifluoride etherate, the benzoylated L-uridine was subjected to alkali-blocking. L-cytidine and L-guanosine were obtained from the silylized heterocycles and from the peracylated pentose as described by Vorbrüggen et al. (Chem. Ber. 114 (1981), 1234-1255) for D-nucleosides, whereby the 2-N-acetyl-6-O-diphenylcarbamoylguanine was used for the synthesis of L-guanosine in analogy to Zou and Robins (Can. J. Chem. 65 (1987), 1436-1437). In order to depict peracylated pentose, the L-ribose was at first obtained from the L-arabinose by epimerization according to Abe et al. (Chem. Pharm. Bull. 28 (1980), 1324-1326) and subsequently derivatized to the 1-O-acetyl-2,3,5-tri-O-benzoyl-β-L-ribofuranoside in a three-stage synthesis, as described for D-isomers by Recondo and Rinderknecht (Helv. Chim. Acta 42 (1959), 1171-1173). In order to prepare the phosphoramidites for solid phase synthesis (FIG. 6), the exocyclic amino groups of the nucleosides were protected by benzoylation of adenosine and cytidine according to Ti et al. (J. Am. Chem. Soc. 104 (1982), 1316-1319) and isobutyrylation according to Flockerzi et al. (Liebigs Ann. Chem (1981), 1568-1585), as described for D-nucleosides. The base-protected nucleosides and uridine were transformed into their 5′-O-dimethoxytrityl-2′-O-triisopropylsilyl derivatives in analogy to Usman et al. (J. Am. Chem. Soc. 109 (1987), 7845-7854). The derivatives were then transformed into their respective 3′-O-(β-cyanoethyl-N,N-diisopropyl) phosphoramidites, according to Milecki et al. (Nucleosides Nucleotides 8 (1989), 463-474). Carrier-bound protected nucleosides were produced (FIG. 7) as described for the D-isomers by Usmann et al. (J. Am. Chem. Soc. 109 (1987), 7845-7854). For the chemical solid phase synthesis of the L-oligoribonucleotides, use was made of equipment of the company Applied Biosystems (model 391 PCR-MATE™ EP and model 394). The syntheses were carried out at a scale of 0,2 μmol by the DNA standard cycle (FIG. 8), whereby the coupling step was extended to 15 minutes. After synthesis, the carrier-bound, protected L-oligoribonucleotide was incubated in 1 ml of a 3:1 mixture of 32% ammonia and ethanol (v/v) for 24 hours at 55° C., in order to separate the oligonucleotide from the carrier and to cleave the protecting groups. The solution was taken off and the carrier material was washed with 400 μl of a 1:1 mixture of ethanol and water (v/v). A 5 μl aliquot was taken from the collected supernatants and UV absorption at 260 nm was determined. Subsequently, the sample was concentrated for drying. In order to cleave the 2′-hydroxyl-protecting group, the oligoribonucleotide was incubated in (10×A260) μl tetrabutyl ammonium fluoride (1.1 M in tetrahydrofurane) and diluted (1×A260) μl ethanol-water (1:1, v/v) and incubated for 72 h at RT. The superfluous tetrabutyl ammonium fluoride was removed by a tip 500 column (QIAGEN). Therefore, the sample was diluted to a total volume of 10 ml by means of 0.1 M triethyl ammonium acetate (TEAAc), pH 7.0 and put on the column equilibrated with 0.1 M TEAAc, pH 7.0. The column was washed twice with each 30 ml 0.1 M TEAAc, pH 7.0 and the sample was then eluted with 10 ml 2 M TEAAc, pH 7.0. The eluate was concentrated for drying and purified by denatured polyacrylamide gel electrophoreses. The product band was detected and excised by UV shadowing. The L-oligoribonucleotide was eluted from the gel piece with H2O for 10 hours and the eluate was desalted with an NAP™10 column (Pharmacia).