The growth rate of Ehrlich ascites tumors has been examined as a function of the zinc content of the diet of the host mice. Imposition of a diet containing a low amount of zinc (1 microgram/g) on the day of tumor transplant leads to a marked retardation in growth. Pretreatment of the mice with this diet slows the growth further so that the lifetime of the mice can be doubled. Cells in such animals are still viable and rapidly proliferate after the animals are placed on a diet containing zinc. Growth rate of the tumor is also recorded at levels of zinc (40, 80, 160, and 250 microgram/ml) in the drinking water. All of these results are examined in relationship to the zinc in the ascites fluid, which provides the zinc nutriture for the tumor. A direct correlation between growth rate and fluid zinc content is observed. The influence of diet and the tumor upon zinc content of the liver of the host is examined. The results indicate that the tumor essentially sequesters zinc from the animal under zinc-deficient conditions. Over a 10-fold range of fluid zinc values, there are no clear differences in the concentration of zinc within the ascites cells. This occurs despite the facile uptake and efflux of zinc ion by the Ehrlich cell.