The t(9,22) chromosomal translocation generating the Philadelphia chromosome and the BCRABL oncogene has been shown both cytogenetically and molecularly to be the etiologic event in chronic myelogenous leukemia (CML). We have designed a ribozyme to cleave the BCRABL mRNA by targeting a GUU triplet adjacent to the junction of the c-BCR and c-ABL fused genes. This ribozyme efficiently cleaved BCRABL RNA transcripts as demonstrated by in vitro cleavage reactions. To determine the effect of constitutive expression of the ribozyme on the elimination of the BCRABL gene product, the ribozyme cDNA sequence was inserted into different retroviral expression vectors. Introduction of the recombinant retroviruses into the CML blast crisis cell-line K562, resulted in the elimination of the P210 protein-kinase activity in several single cell clones infected with the ribozyme expression cassette. Therefore BCR-ABL specific ribozymes may provide a potential genetic therapy for CML.