A spontaneous mammary carcinoma in a high-leukemic AKR strain of mice was studied both in vivo and in vitro, and an attempt was made to elucidate the mechanism of sarcomatous transformation. The tumor, originally a type B adenocarcinoma, transformed into a sarcomatous tumor during serial transplantation in isologous mice. Cell strains were established in vitro from transplants before and after the sarcomatous change. The culture cells were composed of epithelial and flbroblastlike cells and their intermediate forms. It has ceen shown that 1) the carcinoma cells assumed spindle shapes in loosely growing areas, 2) in the mixed carcinosarcomatous stage, the epithelial tumor cells often merged with the stromal cells, indicating a morphological transition from polygonal to elongated cells, and 3) the behavior and morphology of the culture cell strains were similar, although they were derived from histologically different transplants. These findings suggest that the sarcomatous transformation in a serially transplanted AKR mammary tumor resulted from a morphological alteration of the carcinoma cells.