Factors that might affect the biodistribution and clinical utility of radiolabeled lymphocytes were evaluated in experimental animals. Indium-111 (In-111) labeled lymphocytes (10(7)-10(9) syngeneic or allogeneic cells; 1-10 microCi [.037-.37 MBq]/10(8) cells) obtained from peripheral blood, lymph node, or spleen were found in significant amounts in the lymphoid tissues of Lewis rats as early as 3 hours after infusion. A progressive increase in nodal activity with concomitant fall of activity in other organs followed, indicating active recirculation of the lymphocytes. However, In-111 labeled thymocytes or xenogeneic lymphocytes failed to accumulate in lymphoid tissue. In vitro irradiation of the In-111 labeled lymphocytes (100-400 rads [1-4 Gy]) before in vivo administration and increase of the In-111 to 40 microCi [1.48 MBq]/10(8) lymphocytes resulted in no detectable lymphocyte recirculation and/or reduced localization in lymphoid tissue. Splenectomized animals and those sensitized to an organ allograft before cell infusion showed increased activity in their bone marrow. These results suggest that the source of the injected cells, cell irradiation dose level, and host sensitization should be considered when radiolabeled lymphocytes are being prepared for use in clinical diagnosis and therapy.