The role of IF2 from Escherichia coli was studied in vitro using a system for protein synthesis with purified components. Stopped flow experiments with light scattering show that IF2 in complex with guanosine triphosphate (GTP) or a non-cleavable GTP analogue (GDPNP), but not with guanosine diphosphate (GDP), promotes fast association of ribosomal subunits during initiation. Biochemical experiments show that IF2 promotes fast formation of the first peptide bond in the presence of GTP, but not GDPNP or GDP, and that IF2-GDPNP binds strongly to post-initiation ribosomes. We conclude that the GTP form of IF2 accelerates formation of the 70S ribosome from subunits and that GTP hydrolysis accelerates release of IF2 from the 70S ribosome. The results of a recent report, suggesting that GTP and GDP promote initiation equally fast, have been addressed. Our data, indicating that eIF5B and IF2 have similar functions, are used to rationalize the phenotypes of GTPase-deficient mutants of eIF5B and IF2.