The effect of monovalent cation concentrations on the translation was examined in the rabbit reticulocyte cell-free system. The translation of standard reporter gene luciferase was studied using different concentrations of LiCl, NaCl, KCl, RbCl, CsCl, NH4Cl, and (CH3)(4)NCl and the acetates of Na+, K+, and NH4+. Only the salts of K+, Rb+, and NH4+ and to some minor extent of Os' significantly supported translation. Optimum concentrations were dependent on the cation used. Optimum concentrations ranged between 40 mM (NH4Ac), 80 mM (KCl, NH4Cl), and 100 mM (RbCl, KAc). The maximum efficiency of translation depends on the ionic radius of the cation used, KCl and RbCl were superior to all other salts tested in stimulating in vitro translation. The results were confirmed, using a second reporter system, M-hirudin. Here, however, broad optima were observed with RbCl being slightly superior to KCl in supporting translation.