Twenty two kinds of strains and substrains of mammalian cells were cultivated on a gyratory shaker to obtain cell aggregates. The relationship between the morphology of aggregates and tumorigenicity of the cells was investigated. In some groups of cells, the size of aggregates corresponded with their tumorigenicity, e.g. among rat liver cells untreated, hepatoma cells produced by back-transplantation of rat liver cells after the treatment with a chemical carcinogen in tissue culture, and the hepatoma cells passaged serially through rats. The correspondence, however, was generalized to not all the cell strains. It was impossible to identify the tumorigenicity of cells by their aggregate-forming capacity.