The mammalian mitochondrial complex consisting of elongation factors EF-Tu and EF-Ts (EF-Tu.Tsmt) is capable of efficiently binding aminoacyl-tRNA to the ribosome in the presence and absence of guanine nucleotides. In the presence of GTP the binding reaction is catalytic. In the absence of guanine nucleotides, or in the presence of a non-hydrolyzable GTP analog, only one round of ribosome binding occurs. EF-Tu.Tsmt is capable of forming a ternary complex with GTP and Escherichia coli Phe-tRNA as demonstrated by gel filtration chromatography, nitrocellulose filter binding, and by protection of the aminoacyl-tRNA bond from hydrolysis. GDP and the non-hydrolyzable GTP analog guanyl-5'-yl imidodiphosphate are also capable of facilitating ternary complex formation with EF-Tu.Tsmt, but are less effective. No kinetic advantage results from the formation of this ternary complex prior to ribosome binding, and EF-Tu.Tsmt may actually bind aminoacyl-tRNA directly to the ribosome prior to binding GTP. These results suggest that a variation of the prokaryotic elongation cycle is occurring in animal mitochondria. N-Ethylmaleimide inhibits the activity of EF-Tu.Tsmt in polymerization and in ribosome binding. However, the activity of the EF-Tsmt which can be measured independently, is not altered.