Our earlier data, showing the ribosome to be a GEF for RF3 [9], prompted us to re-check the binding of free EF-G to GDP or GTP. The dissociation constant (KD) for the EF-G•GDP complex was about 9 M (Figure 2a), close to an earlier estimate of 4 M [12]. Results from experiments in which [3H]-GDP in complex with EF-G was chased with unlabeled and further purified GTP [9] (see below and Figure 4a for purification details), however, show a 60-fold larger effective KD-value for the binding of EF-G to GTP than to GDP (Figure 2b). This factor of 60 provides a lower boundary to the correct value, because purified GTP solu- tions do contain some fraction of GDP from the hydrolysis of GTP. The intracellular GTP:GDP ratio has been estimated as 7:1 for Salmonella enterica serovar Typhimurium [13], and is probably similar in E. coli. This suggests that a major frac- tion of free EF-G in E. coli is bound to GDP.