This study deals with the biodistribution of syngeneic radiolabeled lymphokine-activated killer (LAK) cells in Wag rats after infusion via the hepatic artery or the jugular vein. The biodistribution of 111Indium-labeled LAK cells was evaluated using serial whole-body gamma camera imaging. Furthermore, we investigated 2 factors that might influence the biodistribution of these effector cells: purity of LAK cells and administration of interleukin-2 (IL-2). After injection of 111Indium-labeled LAK cells via the hepatic artery or via the jugular vein we could detect important differences in the biodistribution pattern up to 5 hr after injection. LAK cells administered via the jugular vein were all found in the lungs up to 2 hr after injection and then redistributed to the liver and to the spleen. LAK cells administered via the hepatic artery were all found in the liver after injection and redistributed after 2 hr mainly to the spleen. About 8 hr after injection we could no longer detect any differences in the biodistribution pattern according to the route of administration. Biodistribution was followed for up to 72 hr after injection but the pattern showed no change after 8 hr, whichever the route of administration. A purified adherent-LAK population, a large granular lymphocyte culture with only 6% T cells, showed the same distribution pattern as standard LAK cells (40% T cells). Infusions of 40,000 units of IL-2 per day, starting 3 days before and continuing after administration of radio-labeled LAK cells, accelerated the redistribution of these cells by both routes of administration. We conclude that up to 2 hr after local infusion, a high concentration of LAK cells in the first capillary bed can be obtained. Therefore, local administration of LAK cells may be more effective against tumors.