Abstract Escherichia coli ribosome released protons upon addition of Mg 2+. The Mg 2+-induced proton release was studied bv means of the pH-stal technique. The number of protons released from a 70 S ribosome in the Mg 2+ concentration range 1–20 mM was about 30 at pH 7 and 7.6. and increased to about 40 at pH 6.5. The rRNA mixture extracted from 70 S ribosome showed proton release of amount and of pH dependence similar to those of the 70 S ribosome but the ribosomal protein mixture released few. This indicates that rRNA is the main source of the protons released from ribosome. The pH titration of rRNA showed that the p K a values of nucleotide bases were downward shifted upon Mg 2+ binding. This p K a shift can account for the proton release. The Scatchard plots of proton release from rRNA and ribosome were concave upward, showing that the Mg 2+-binding sites leading to proton release were either heterogeneous or had a negative cooperativity. A model assuming heterogeneous Mg 2+-binding sites is shown to be unable to explain the proton release. Electrostatic field effect models are proposed in which Mg 2+ modulates the electrostatic field of phosphate groups and the potential change induces a shift of the p K a values of bases that leads to the proton release. These models can explain the main features of the proton release.