To exhibit physical properties and characteristics of an aqueous solution of magnesium chloride in the body of a plant and in a vegetable food, activate the antibody production in the body, prevent diseases and insect pests by an improvement in immunity and surely and smoothly grow a plant.

This method for activating the antibody production of a plant, etc., comprises utilizing a means such as dipping of a plant, etc., in an aqueous solution of magnesium chloride, spraying of the aqueous solution or water supply, activating immunopotentiating actions in the body of a plant, etc., and activating the plant, etc. Thereby, the activation of the antibody production (immunopotentiating actions in the body) is promoted by grasping the specificity of the magnesium chloride and utilizing the bacteriostatic or fungistatic properties of the naturally produced magnesium chloride and physical properties of the magnesium chloride retaining a constant composition thereof for water. The improvement in immunity and sure and smooth growth can be carried out. The method has a good influence on a living thing (a human, an animal, etc.) eating a vegetable food, etc.