We developed a novel magnetic resonance (MR) imaging contrast agent, MAb-magnetite, that was prepared by covalently linking polyethylene glycol-coated magnetite to a monoclonal antibody specific for a human glioma cell-surface antigen. When MAb-magnetite was injected intravenously into tumor-bearing nude mice at a dose of 100 mumol Fe/kg body weight, a 50% decrease in the T2 signal intensity of the tumor was observed, immediately following administration and continued for 48 h. Microscopic observation of the tumor tissue demonstrated localization of MAb-magnetite in cancer cells. These results suggest that MAb-magnetite is a promising agent for MR imaging of neoplasms.