At low NH4+ concentrations, 50S ribosomal subunits from E. coli were fully active in the absence of 30S ribosomal subunits, in forming a complex with the polypeptide chain elongation factor G (EF-G) and guanine nucleotide (ternary complex formation), and also in supporting EF-G dependent hydrolysis of GTP (uncoupled GTPase reaction). However, both activities were markedly inhibited on increasing the concentration of the monovalent cation, and at 160 mM NH4+, the optimal concentration for polypeptide synthesis in a cell-free system, almost no activity was observed with 50S ribosomes alone. It was found that the inhibitory effect of NH4+ was reversed by addition of 30S subunits. Thus, at 160 mM NH4+, only 70S ribosomes were active in supporting the above two EF-G dependent reactions, whereas at 20 mM NH4+, 50S ribosomes were almost as active as 70S ribosomes. Kinetic studies on inhibition by NH4+ of the formation of 50S ribosome-EF-G-guanine nucleotide complex, indicated that the inhibition was due to reduction in the number of active 50S ribosomes which were capable of interacting with EF-G and GTP at higher concentrations of NH4+. The inhibitory effects of NH4+ on ternary complex formation and the uncoupled GTPase reaction were markedly influenced by temperature, and were much greater at 0° than at 30°. A conformational change of 50S subunits through association with 30S subunits is suggested.