A method for restoring immunoreactivity of a tissue, particularly decalcified tissue, fixed width an aldehyde fixative agent and embedded in an embedding medium, usually comprising celloidin, the method comprising the steps of contacting the tissue with an aldehyde releasing reagent solution comprising a solvent and an aldehyde releasing reagent and removing aldehyde released by the aldehyde releasing reagent from contact with the tissue by reacting the aldehyde in a substantially irreversible manner to form a non-aldehyde derivative, and removing excess base from the tissue. A preferred solution for celloidin-embedded decalcified tissue comprises methanolic sodium hydroxide at about one-third saturation.