Mice injected iv with a suspension of Lewis lung carcinoma cells were divided into three groups: untreated; treated ip with levan from Day -8 until Day 0; treated with levan from Day -8 until death. Survival was longest and loss of weight lowest in mice levanized until Day 0 and moderate in the continuously levanized group in comparison to untreated mice. The treatment failed, however, to cure any animals. Histological study revealed levan-filled foamy macrophages in the metastases of levan-treated mice. There was also evidence of lymph stasis with severe dilatation of lymphatics and deranged spread of the tumors along lymphatic channels. Electron microscopy suggested less adhesion of tumor cells to vessel walls in the levan-treated mice. The findings suggest that treatment with levan might delay transfer of cells from vascular lumina and therefore expose them for longer periods to the intravascular host defense mechanisms and to activated macrophages. No explanation is offered for the longer survival of mice treated only until Day 0 in comparison to the continuously treated animals.