The limitations of conventional treatment for non-Hodgkin's lymphoma (NHL) have led to intense investigations of biological therapies for this disease, including monoclonal antibodies. Conjugation of these antibodies with radionuclides results in additional cytotoxic activity with generally acceptable levels of toxicity. Numerous antibodies and radionuclides of widely varying properties are in clinical development. Given the likelihood that one or more radioimmunotherapeutic agents will soon be commercially available, there is a need for oncologists who treat patients with NHL to become more aware of the principles of radiation physics that underlie radioimmunotherapy and the interactions among radiolabeled antibodies, the tumor, and the host.