Oligoribonucleotides containing four G clusters, (UGnU)4 (n = 3-5), were synthesized and characterized by electrophoretic analysis and resistance to nucleases. Electrophoretic analyses indicate that the oligomers except for (UG3U)4 can form antiparallel-stranded monomeric quadruplexes in the presence of potassium cation. The oligomers used in this study are protected from endonuclease P1 and venom phosphodiesterase degradation. In addition, the oligomers indicate resistance to nucleases in human serum and E. coli S-100 fraction.