The establishment of two syngeneic mouse sarcoma lines is described, which were induced with benzo[a]pyrene originally as solid tumours growing in the subcutaneous region, and which subsequently changed to the ascitic form. Successive i.p. transplantations produced progressive increases of malignancy. The same progressive tendencies were found for metastatic trials carried out in parallel with the i.p. transplantations. The use of both ascitic tumour cell lines for standardized chemotherapeutical screening systems is discussed.