Ribosomal proteins situated at or near the binding site of elongation factor G (EF-G) on the Escherichia coli ribosome have been identified by use of the bifunctional, cleavable cross-linker, dimethyl-4,9-diaza-5,8-dioxo-6,7-dihydroxy-dodecanebisimidate. Five different bimolecular EF-G x ribosomal-protein complexes were isolated electrophoretically. The ribosomal proteins found in each of these complexes were identified as the 50-S-subunit proteins L6, L7/L12 and L14 and the 30-S-subunit proteins S12 and S19. In the presence of thiostrepton, which prevents binding of EF-G to the ribosome, there was a considerable decrease in the yield of each of these cross-linked complexes. The data suggest that EF-G is bound close to the ribosomal subunit interface.