¹²⁵I-5-iodo-2′-deoxyuridine was used to label pure B16 melanoma cells grown in tissue culture. In vitro and in vivo studies established the optimal, nontoxic labeling dose. C57BL/6J mice were given intravenous injections of labeled cells and killed at various intervals after injection. Blood and organs were collected, processed, and monitored. The label was released only after cell death. It was poorly reutilized and quickly eliminated. Hence, organ radioactivity represented almost exclusively the number of live cells present when the animals were killed. The data permitted the following conclusions: The lung contained most of the tumor cells at all intervals. Some tumor cell emboli recirculated. Tumor cells in the lung decreased in numbers at about 5 minutes post injection and returned to high levels by 10 minutes post injection. Tumor cells died rapidly, and about 1% of cells survived after 24 hours. About 400 melanoma cells were in the lung after 14 days, and those yielded an average of 78 metastases. Apparently very few surviving tumor cells are needed to establish a metastasis.