Mouse-spleen lymphocytes, mouse mammary adenocarcinoma cells (MMA-67) and mouse lymphoma cells (S49.1) were radiolabeled with 111In and their in vivo migration patterns studied over a 24-72 h period after i.v. injection into syngeneic mice. All three cell lines showed different in vivo migration patterns. Initially, some trapping of the cells occurred in the lungs at 15 min. More MMA-67 cells were initially trapped in the lungs than S49.1 cells or lymphocytes. At 24 h, most of the 111In labeled cells had left the lungs and accumulated in various tissues and organs. At 48 and 72 h, only slight changes in the localization patterns of the 111In radiolabeled cells in vivo were noted when compared to the 24 h patterns. About 35% of the injected 111In was excreted from the animals over the 72 h period of these experiments. In vitro studies compared the growth and/or viability of 111In radiolabeled lymphocytes or tumor cells to nonradiolabeled lymphocytes or tumor cells. The percentage of 111In released from the cells in culture was also determined. Similar values for growth and/or viability were obtained between non-radiolabeled cells and MMA-67 cells radiolabeled with 1.35 dpm 111In per cell; S49.1 cells, 0.51 dpm111In per cell; and lymphocytes, 0.04 dpm 111In per cell.