Disclosed herein are isolated monoclonal human antibodies that specifically binds insulin-like growth factor II (IGF-II) with an equilibrium dissociation constant (Kd) of 1 nM or less, wherein the antibody bind IGF-I with an equilibrium dissociation constant (Kd) of 1 mM or greater. The antibodies inhibit phosphorylation of the insulin-like growth factor receptor. Nucleic acids encoding these antibodies, expression vectors including these nucleic acids, and isolated host cells that express the nucleic acids are also disclosed. The antibodies can be used to detect human IGF-II in a sample. Methods of diagnosing a tumor are disclosed herein that utilize these antibodies. Methods of treating a subject with a tumor are also disclosed.