Amino acid sequences were obtained for four peptides (p1, -2, -3 and 4) generated by chemical or proteolytic cleavage of a 25 kDa GTP-binding protein purified from human placental and platelet membranes. The peptides shared sequence similarities with those contained in several of the ras-related GTP-binding proteins. Peptide p2, a 12-mer, was homologous with a region of the GTP-binding proteins that contains a structural motif proposed to contribute to the nucleotide binding site. However, whereas nearly all GTP-binding proteins exhibit the residues NKXD as this motif, p2 contains TQID. Antisera (Ap1 and Ap3) raised against synthetic peptides corresponding to p1 and p3 specifically reacted on Western blots with the 25 kDa GTP-binding protein purified from human placenta, human platelet and bovine brain as well as with a 25 kDa polypeptide in various cell lines. These results demonstrate the widespread existence of an abundant 25 kDa GTP-binding protein which contains a putative nucleotide binding domain that is chemically distinct from that described for all GTP-binding proteins of known primary structure.