The interactions of yeast tRNAAsp with cognate aspartyl-tRNA synthetase have been studied in high concentrations of either sodium chloride or ammonium sulphate by fluorescence titration and small-angle neutron scattering. In solutions containing more than 1M NaCl no complex is formed and enzymatic activity is abolished. In strong contrast, however, the physical measurements showed the formation of a two-to-one tRNA-enzyme complex, with high affinity, in 1.6 M (NH4)2SO4. Aminoacylation assays under the same salt conditions showed the enzymatic fixation of aspartic acid to tRNAAsp to occur at an appreciable rate. The present study emphasizes that the effects of salts on protein-nucleic acid interactions do not depend only on ionic strength but also on the nature of the salt. This study has allowed a rational approach to the crystallisation of a functional tRNAAsp-aspartyl-tRNA synthetase complex (Giegé, Lorber, Ebel, Thierry and Moras (1980) C.R. Acad. Sci. Paris, série D, 291, 393-396).