Liposomes containing ultrafine magnetite were prepared and their transport characteristics in a magnetic field were examined in vitro and in vivo. About 15μg of magnetite (Fe₃O₄) could be entrapped in the liposomes per μmol of lipid without impairing the liposomal encapsulation capacity for the aqueous marker (³H-inulin). In in vitro experiments, when 0.1 ml of liposome suspension (0.86μmol of lipids and 13.3μg of magnetite) was applied to a glass capillary at a flow rate of the medium of 0.17 ml/min, about 35% of the liposomes was found in the region of the applied magnetic field (4000 G).It became apparent that the ability of the magnetic field to hold the liposomes was influenced by the flow rate of the medium and the magnitude of the magnetic field. The viscosity of the medium and diameter of the capillary where the magnet is applied for fixation of the liposomes may also affect the holding capacity.In vivo experiments were carried out with rats having Yoshida sarcoma implanted in a foot pad. The liposomes were injected from the branch of the femoral artery without impairing normal blood flow in the tumor tissue. A very small amount of the liposomes, but significantly more than the control, was trapped at the tumor tissue.The trapped amount of liposomes in the present expriments appeared to be insufficient for practical therapy, but a more powerful magnet may give better results.