Clinical studies using indium-111 oxine labelling of human peripheral blood lymphocytes are presented. Data from animal models of lymphocyte migration are compared with results found in healthy subjects and patients with malignant neoplasms. The physiological significance of bone marrow and liver localization on gamma camera imaging is discussed and the importance of considering the surface marker characteristics of the lymphocytes under study, when interpreting results, is emphasized. The possibility that the redistribution of lymphocytes within the body is a cause of the peripheral blood lymphopenia in patients with Hodgkin's disease and other malignancies is suggested, and the usefulness of indium-111 oxine labelling in clarifying this problem is proposed.