This report is concerned with morphological studies on tissue culture cells derived from a spontaneous mammary carcinoma and it's serially transplanted tumors of AKR mice. Four lines of tissue culture cells were obtained. A-419 and A-319 lines of cells were obtained from the fourth transplant generation of the tumor (adenocarcinoma), A-600 line of cells were obtained from the 26th transplant generation of the tumor (sarcomatous), and A-162 line of cells were from the adenocarcinoma produced by back-transplantation of primary culture cells of A-419 line cells to AKR mice. These four lines of tissue culture cells showed similar morphology and distribution, mixed growth of both epithelial cells and fibroblastic cells. Back-transplantation of primary culture cells of A-419 and A-319 lines to AKR mice produced adenocarcinomas, but A-319 line cells of later passages (28th, 29th, 31st, and 35th passage) and A-600 line cells produced sarcomas when transplanted back to AKR and C(3)Hf mice. Two mechanisms have been proposed to explain the sarcomatous transformation; 1) transformation of carcinoma cells to sarcoma cells and 2) malignant change of interstitial connective tissue cells during serial passages of the culture cells. The second mechanism was considered to be most probable in the present case.