PURPOSE: To improve sensitization efficiency by adding an oily material liquid consisting of polyhydric isocyanate and an oily material to an aq. soln. contg. polyvinyl alcohol PVA and polystyrene sulfonic acid PSS and polymerizing the same.

CONSTITUTION: The microcapsule of this invention is formed in the following manner: Oil-soluble polyhydric isocyanate, isothiocyanate or the prepolymer thereof is dissolved in an oily material forming a core to prepare an oily material liquid and an aq. soln. (aq. PVA-PSS soln.) formed by dissolving both of polyvinyl alcohol PVA and polystyrene sulfonic acid PSS is prepd. The above-described oily material liquid is added to the aq. PVA-PSS soln. and is emulsified and dispersed so as to coexist in the PVA and PSS. A polyhydric amino compd. is added to said emulsified dispersion and is heated at about 70°C for about 2hr to effect thermal polymerization. The resulted microcapsule exhibits extremely high detection sensitivity in agglutination when used as a microcapsuled reagent sensitized with an antigen or antibody.