A lactoferrin partial peptide or lactoferrin hydrolyzate mixture that can be obtained by hydrolysis of lactoferrin with the use of a hydrolase and that in the antibody therapy for cancer, have the efficacy of enhancing the cytotoxic activity of antibody drug. The lactoferrin partial peptide or lactoferrin hydrolyzate mixture can be used as an active ingredient of medicine for enhancing the cytotoxic activity of antibody drug in the antibody therapy for cancer.
