In order to determine the mode of action of cytostatic 9-beta-D-xylofuranosyladenine (xylo-A), the inhibitory effects of 9-beta-D-xylofuranosyladenine 5'-triphosphate (xylo-ATP) on DNA-dependent RNA polymerases I and II purified from cherry salmon (Oncorhynchus masou) liver nuclei were studied. This nucleotide showed strong inhibitory action on both RNA polymerases I and II. The K1 values are 14 microM for polymerase I and 5 microM for polymerase II (Km values of ATP are 37 microM for polymerase I and 40 microM for polymerase II). The mode of xylo-ATP was competitive with respect to the incorporation of AMP into RNA and non-competitive to UTP and CTP.