[Problem] To provide an agent for enhancing the immunological functions of another animal in which antibodies and immunocytes do not pass from a mother to a fetus across the placenta, said agent exhibiting few side-effects, and particularly to provide an agent for treating frailty syndrome in newborn animals. [Solution] The problem is solved by an agent which includes, as an active ingredient thereof, activated lymphocytes derived from the tissue of an animal in which antibodies and immunocytes do not pass from a mother to a fetus across the placenta, and which is used to enhance the the immunological functions of another animal of the same species as the animal from which the lymphocytes are derived. The present invention also provides a production method for an agent for enhancing the immunological functions of another animal, wherein lymphocytes separated from an animal are grown and activated to produce activated lymphocytes.
