Chronic myelogenous leukemia (CML) is associated with a translocation of the BCR and the ABL genes, t(9;22). Results of this event are transcription and translation products that are unique to malignant cells. We therefore designed synthetic ribozymes which are capable of exclusively cleaving the BCR/ABL B3A2-type mRNA without altering normal cellular transcripts. Synthetic B3A2-type transcripts could only be cleaved by B3A2-type mRNA targeted ribozymes and not by any of the controls. The B3A2-type mRNA directed ribozyme, on the other hand, did not cleave any of the control transcripts. The effective delivery of ribonucleic acids by lipofection into K562 cells could be demonstrated by fluorescent microscopy, slot blot analysis, and RNA polyacrylamide gel electrophoresis. In vivo, we were able to induce a significant inhibition of the proliferation of K562 cells with ribozymes directed against the B3A2-type mRNA. Quantitative PCR analyses showed an up to fivefold reduction of the relative number of BCR/ABL mRNA molecules per single cell after exposure to ribozymes compared to controls. We conclude that ribozymes targeted against the B3A2-type BCR/ABL mRNA function in vitro and in vivo.