A monoclonal antibody (MAb) to infectious bursal disease virus (IBDV) that has virus-neutralizing activity was bound to polystyrene latex microspheres The microspheres agglutinated with extracts of bursae from chickens infected with IBDV. Agglutination was inhibited in a competitive manner by adding serum obtained from IBDV-infected chickens. The level of agglutination-inhibition depended on the serum antibody titer against IBDV. The reaction was visually accomplished within 5 minutes. The titer of this rapid assay showed a close relationship with that of the virus-neutralization test.