Chinese hamster V79 lung cells grown in suspension in tissue culture form multicell spheroids morphologically resembling the nodules in syngeneic transplants of the C3H mammary carcinoma in mice and a number of carcinomas in patients. The growth curve of the cell spheroids is similar to that of the transplants in the mouse. On microscopic examination they develop an outer zone containing many cells in division, an intermediate zone which is poorly nourished and oxygenated and contains few cells in mitosis, and a central zone of necrosis. The spheroids can be broken up by trypsinization and the single cells assayed for colony formation. They provide a useful in vitro model for assessment of effects of nutrition and oxygenation on growth and for determination of growth fraction after treatment with drugs or radiation.