An object of the present invention is to provide a reagent for activating and unmasking antigens in a immuno-tissue-chemical staining. A slice of tissue cells on a slide glass is heated or heated under pressurization by an electric pot, autoclave, microwave oven, scalder, or constant-temperature bath for a sufficient time period in CCA aqueous solution of suitable pH and CCA concentration, thereby activating and unmasking antigenicity of antigen masked by aldehyde fixation.