Microprotein-inactivating ultrafine metal particles comprising ultrafine metal particles having a bond between an organic acid component and a metal, having an infrared absorption peak ascribable to the bond between the organic acid and the metal near 1518 cm−1, and capable of efficiently inactivating microproteins without impairing properties of the resin or formability thereof. When used in the form of a resin composition or a coating agent, the microprotein-inactivating ultrafine metal particles are capable of exhibiting an effect for inactivating microproteins.
