A lactoferrin or lactoferrin hydrolyzate antimicrobial activity enhancing agent in which the active ingredient is ribonuclease-4, as well as a method for producing a lactoferrin or lactoferrin hydrolyzate antimicrobial activity enhancing agent, the method being characterized in comprising: a step (a) for bringing a crude dairy material into contact with a cation-exchange resin to adsorb protein in the crude dairy material to the cation-exchange resin； a step (b) for washing the cation-exchange resin after the adsorption, and passing the same through an elution solvent to elute a fraction having an isoelectric point of 8.0 to 10.0； and a step (c) for subjecting the eluted protein fraction to gel filtration using a gel filter carrier, to recover a fraction having a molecular weight of 30 kDa or lower.
