Magnetic nanoparticles will heat when activated by an alternating magnetic field (AMF). Systemic delivery of magnetic nanoparticles conjugated to a monoclonal antibody (MAb) offers potential for selective and minimally invasive cancer treatment by heating the microenvironments of cells containing such particles. 111In-Chimeric L6 (Ch L6) antibody was conjugated to carboxylated polyethylene glycol (PEG-COOH) on dextran-coated iron oxide 20-nm particles to yield bioprobes. Pharmaco-kinetics, histopathology, and AMF/bioprobe therapy were done using intravenous doses (20μg/2.2mg ChL6/ bioprobe) with 50μg ChL6 in athymic mice bearing human breast tumor, HBT3477, xenografts. A 153 kHz AMF was applied 72 hr post injection for therapy using several AMF amplitudes. Significant therapeutic response was demonstrated. Toxicity was observed only at extreme AMF conditions, and was due to nonspecific heating resulting from the AMF.