Ribozymes are effective tools for the cleavage of target RNAs in a sequence-specific way. In chronic myelogenous leukemia (CML) the reciprocal translocation of chromosomes 9 and 22 results in the formation of the unique BCR/ABL fusion gene which is believed to play a crucial role in the establishment of CML. In order to decrease the BCR/ABL gene product we designed short synthetic ribozymes and analyzed their effects on the proliferation rate of K562 cells. Ribozymes proved to have a higher inhibitory potential as conventional antisense constructs of comparable nucleotide sequence.