Abstract: Methods and devices for selectively removing from a subject a target cell, pathogen, or virus expressing a binding partner on its surface are presented. In one embodiment, the device contains an excorporeal circuit, which includes, at least, a magnetic filter comprising a magnet and a removable, magnetizable substrate capable of capturing magnetic nanomaterials; and a pump in fluid communication with the magnetic filter, wherein the pump moves fluid through the excorporeal circuit. The magnet is capable of generating a magnetic field sufficient to capture magnetic nanomaterials in the magnetic field. In a preferred embodiment, the target cells are cancer cells and/or cells infected with pathogenic agents. The devices may be designed for extracorporeal or in vivo uses. Functionalized superparamagnetic nanoparticles are either mixed ex vivo with a biological fluid from the patient or injected into the patient.