By means of the tissue culture technic it is possible to study the individual cells which comprise a neoplastic growth and to identify certain cells that are peculiar to that growth. These are in all probability the tumor or malignant cells. A number of investigators have availed themselves of this method and, in this way, the characteristic cells of a number of malignant growths of man, of rats, and of mice have been identified and described. An earlier study was made of two transplanted Cysticercus sarcomata, IRS 1548 and IRS 146, in tissue cultures, and their malignant cells were described. These sarcomata were of the polymorphous-cell type, and the original tumor from which they were derived formed part of a large group of Cysticercus tumors which have been experimentally induced in the livers of rats by feeding these animals the eggs of Taenia crassicollis (Bullock and Curtis). In order to determine whether there existed any essential morphologic differences between the malignant cells of various Cysticercus sarcomata of this group, the present investigation was undertaken. It is concerned with the cultural characteristics and the identification of the malignant cells of two spindle-cell sarcomata, IRS 4337 and 4338. Materials The tumors utilized in this investigation were obtained at the Crocker Institute from a group of rats bearing primary tumors in Cysticercus cysts of the liver. By means of frozen sections, two mixed-cell sarcomata, predominantly spindle-cell, viz. , IRS 4337 and IRS 4338, were selected.