An antimicrobial activity enhancing agent for lactoferrin or a lactoferrin hydrolysate, which contains ribonuclease-4 as an active ingredient, and a method for producing an antimicrobial activity enhancing agent for lactoferrin or a lactoferrin hydrolysate, 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 8.0 to 10.0, and (c) the step of subjecting the eluted protein fraction to gel filtration using a gel filtration carrier to collect a fraction of a molecular weight of 30 kDa or less.
