In chronic myelogenous leukemia (CML) the reciprocal translocation of the long arms of chromosomes results in the formation of the unique BCR/ABL fusion gene which is believed to play a crucial role in the pathogenesis of CML. Different short synthetic ribozyme constructs were compared with regard to their efficiency to cleave the BCR/ABL target RNA. In the CML cell line K562 we were able to inhibit the p210BCR/ABL synthesis by a ribozyme which was about twofold more effective than the corresponding antisense molecule.