PURPOSE: To obtain a low-concn. microcapsuled reagent which can provide high sensitivity in immunoreaction by adjusting the solid content in the microcapsule to a specific wt% range.

CONSTITUTION: A reagent decreases the efficiency in antigen antibody reaction and generates non-spefific agglutination when the solid content as the carrier in said reagent exceeds an upper limit. Conversely, when the solid content is less than a lower limit, the thickness of the layer of the agglutinated lump in the bottom of a microplate tube occuring in the antigen antibody reaction decreases, resulting in decreased contrast and easier tendency to slip-down. Decision of the agglutination is then difficult. The concn. of the sensitized microcapsule to be incorporated into the reaction is therefore selected within a 0.1W0.6wt%, more preferably, 0.2W0.5wt% range of solid content. The sensitized microcapsule is incorporated in the core material circule thereof with a fluorescent material which is excited by UV light to emit light in a visible region. The agglutinated image of the thin color which cannot be decided by the naked eyes is thus made observable in high contrast by the bright fluorescent emission according to this reagent.