A brief account is given of the transformation of seemingly normal rat fibro-blasts into several strains of malignant cells. These conversions are permanent and are characterized by changes in the morphology of the cells; in the pattern of cellular division; and in cultural characteristics of the colonies. That these changes were evidences of the fibroblasts having become malignant was shown by the inoculation of altered cells into rats and the subsequent production of several different types of malignant tumors. Some possible explanations for the occurrence of these changes are considered, but the actual demonstration of a single causative factor is lacking. The question arises what effect, if any, does a malignant cell have upon adjacent benign cells. A method has been devised by Gey for the isolation, without serious trauma, of a single malignant cell. Such cells have been transferred to culture media alongside normal fibroblasts. The division and subsequent multiplication of the malignant cell has been photographically recorded, and a tumor strain has been derived from such a single cell. In the preliminary expts., however, there was no discernible effect upon the fibroblasts.