A series of N2-substituted guanosine 5'-triphosphates was synthesized from the corresponding nucleosides. The nucleosides were prepared by treatment of N2-substituted guanines with tetra-O-acetylribose under conditions which maximized the yield of the 9-beta-guanosine isomers. These nucleotides and several sugar- and base-modified analogues of GTP were tested for their ability to bind to cellular and oncogenic forms of the GTP/GDP binding proteins, Ha-ras-p21. Several N2-substituted GTPs showed affinities higher than that of GDP itself, and the N2-[p-(n-butyl)phenyl] derivative bound to the oncogenic mutant, Leu-61 p21, twice as strongly as to the cellular protein. Changes in relative affinities of the nucleotides are discussed with reference to reported crystallographic structures of p21.