An antimicrobial activity enhancing agent for lactoferrin or a lactoferrin hydrolysate, which contains a milk protein as an active ingredient and has the following properties, and a method for producing the antimicrobial activity enhancing agent, which comprises (a) the step of contacting raw milk with a cation exchange resin to adsorb proteins in the raw milk to the cation exchange resin, (b) the step of washing the cation exchange resin having undergone the adsorption treatment, and flowing an elution solvent to elute a protein fraction having an isoelectric point of 9.0 to 11.0, and (c) the step of subjecting the eluted protein fraction to membrane filtration using an ultrafiltration membrane having a molecular weight cutoff of 30 kDa to prepare the milk protein having the following properties: (A) an action of enhancing the antimicrobial activity of lactoferrin or a lactoferrin hydrolysate, (B) a molecular weight of 30 kDa or less, (C) an isoelectric point of 9.0 to 11.0.
