Association constants (Kc,') for the binding of 50s ribosomal subunit proteins L5, L18, and L25 to the 5s ribosomal RNA of Escherichia coli have been determined by a membrane filter assay. Values for K,'are 2.3 X IO8 M-' for the L18-5s RNA complex, 1.5 X IO7 M-' for the L25-5s RKA complex, and 2.3 X IOh M-I for the L5- 5s RNA complex at 25 “C in TMK buffer (50 mM Tris-HCI (pH 7.6)-20 mM MgC12-300 mM KCI). Although the affinity of L5 increases by approximately one order of magnitude in the presence of LI 8, estimation of K,' was not feasible in the ternary complex. Standard thermodynamic quantities for the individual protein-5S RNA interactions were calculated from the variation of K,' with temperature. Enthalpy and entropy changes both contribute to the free energy of binding in all three cases. Since the enthalpic terms are small, however, it is unlikely that the associations lead to major alterations in the structure of the ribosomal components. Circular dichroism measurements confirm that the 5s RNA undergoes no detectable change in secondary structure as a result of association with L5 or L25. Formation of the L18-5s RNA complex, by contrast, is accompanied by a significant increase in the circular dichroism at 268 nm, suggesting that the protein induces a shift in the configuration of one of the double-stranded regions of the RNA molecule. This observation may help to explain the strong cooperative influence of LI 8 upon the binding of L5 to the 5s RNA.