In-vitro-generated lymphokine-activated killer (LAK) cells of BALB/c mice, bearing the syngeneic colon carcinoma C-26 for 7 days, were as efficient as those from normal mice in lysing C-26 cells whereas LAK cells from 14-day tumor-bearing and 5- and 14-day tumor-resected animals had a lower C-26 cytotoxicity. The level of C-26 lysis returned to normal values 30 days after surgery. To identify the best source of LAK cells in vivo, groups of normal mice were treated with 10(4), 3 x 10(4) or 10(5) U/day of interleukin 2 (EL-2) for 7 days intraperitoneally (i.p.) or intravenously (i.v.) (3 x 10(4) dose only). The highest lysis on C-26 was obtained from peritoneal exudate cells of mice given 3 x 10(4) and 10(5) U whereas spleen cells were lytic only when taken from mice treated with 10(5) U IL-2. Peripheral blood lymphocytes lacked any cytotoxicity except for the group of mice which received IL-2 i.v. The kinetics of in vivo LAK activation in different organs showed a peak of anti-(C-26) lytic activity at day 5 in peritoneal exudate cells and spleen cells of mice given IL-2 for 5 days whereas administration of LAK cells alone had no effect: IL-2 plus LAK cells gave a lower peak of LAK activity as compared with IL-2 alone. A lower level of in vivo LAK activation was found in mice whose tumor was resected 5 days before; such impairment was evident even 14 days after surgery. Homing experiments were carried out with i.v. injected 51Cr-labelled LAK cells in normal or tumor-resected mice. In normal mice the highest radioactivity at 30 min was found in the lungs; liver and spleen also showed high radioactivity whereas blood had a negligible amount of radioactivity. Radioactivity declined rapidly in lungs (less than 10% after 24 h) while remaining at appreciable levels in the liver after 24 h and 48 h; spleen showed constant levels of 12%-15%. Homing of LAK cells was altered in mice receiving IL-2 i.p. for 5 days with slower and lower radioactivity peaks in the lung and higher levels in liver. In tumor-excised mice lower levels of radioactivity were found in lungs. These results show that: (a) alterations in LAK activity occur in early-tumor-resected and large-tumor-bearing animals; (b) the route of IL-2 administration is critical in LAK activation in vivo; (c) treatment with IL-2 modifies LAK homing.