The effects of mono-and divalent metal ions on the pectin estrase (PE) activity and hardness preheated at 60°C for 2 h in aqueous solutions containing sodium, potassium, calcium and magnesium ions were studied. The effect of each metal ion on the PE activity could be expressed more generally by the ionic strength than by the molality and is regarded as being almost the same at an ionic strength corresponding to that in the plant tissue. In view of this, the ionic strength of each salt solution was adjusted to 0.15 mol/kg. When Japanese radish was preheated in aqueous solutions containing the metal ions individually or in deionized water alone, hardening took place to a similar extent with or without metal ions added. The hardness of the sample treated with the divalent metal ions was slightly greater than that treated with monovalent metal ions. The deesterification of pectin by PE was the main cause of this hardening, the effect of binding between the divalent metal ions and pectin not being a major factor. The difference in pH value before and after preheating in an aqueous solution containing the metal ions was slightly greater than that in deionized water. In the aqueous solution containing magnesium ions, the effect of reducing the pH value was slightly greater than that with the other ions. When both the calcium and magnesium ions were removed, the solutions containing the monovalent metal ions showed a greater decrease in hardness than those containing the divalent metal ions. The divalent metal ions seemed to indirectly promote interaction among the tissue components.