Abstract E. coli 50S ribosomal subunits show in the absence of 30S subunits and at low NH 4 + or K + high turnover activity in EF-G-dependent GTP hydrolysis which is inhibited by increasing concentrations of monovalent cations. At 80 mM NH 4 + or K + this activity is already 70–80% inhibited. This effect is reversed by 30S which are stimulatory with an optimum at about 80 mM for NH 4 + and 20–40 mM for K +. At low NH 4 + or K + (<5 mM) stimulation by 30S of maximal 50S activity depends on the [EF-G]/[50S]. Unlike EF-G, EF-T does not show any Phe-tRNA-dependent GTPase activity with 50S alone even at low concentrations of NH 4 + or K +.