Elongation factor Tu (EF-Tu) binds GTP and aminoacyl-tRNA (aa-tRNA) forming a ternary complex which is delivered to the A-site of the ribosome. Animal mitochondrial tRNAs are quite unusual and lack many of the residues important for the stabilization of the structures of other tRNAs. The stabilities of the ternary complexes formed by mammalian mitochondrial and E. coli EF-Tu were determined with four bovine mitochondrial aa-tRNAs. The ternary complex with Phe-tRNA(Phe) has a Kd of about 75 nM. Equilibrium dissociation constants are tightest for the two native Ser-tRNA species (17 nM). Ternary complexes formed with the transcript of tRNA(AGY)Ser are 10-fold weaker than those formed with the native tRNA.