Several human milk proteins have physiologic functions in infants. These proteins are involved in defense against infectious agents and in the optimization of nutrient uptake from milk. Therefore, interest in producing recombinant human milk proteins to use in infant formula has been growing. Microorganisms and transgenic animals can now be used for the production of bioactive proteins. However, the benefits of each protein must be evaluated in cells, animal models, and infants before claims can be made that adding them to formula improves the health or nutrition of infants. Once benefits are shown, proper manufacturing conditions must be developed for introducing the protein or proteins into formula. Processing conditions must be evaluated to ensure that biologic activity is maintained. Dry blending, aseptic processing, sterile filtration, and other techniques will likely be necessary for introducing proteins that require specific tertiary structure for activity. The importance of posttranslational modifications must also be considered: some proteins may require proper glycosylation or phosphorylation for physiologic activity.