The bcr-abl chimeric gene is found in 95% of chronic myeloid leukemia (CML) patients and is thought to be seminal to the etiology of the disease. The possibility of using ribozymes to suppress bcr-abl gene expression and subsequently alter the malignant phenotype of hematopoietic cells may provide an alternative therapeutic approach to current regimens. A series of hammerhead ribozymes targeted to a b3a2 bcr-abl transcript has been developed and previously shown to be capable of cleaving the desired sequence with varying degrees of specificity. This study investigated the ex vivo effects of endogenous expression of these ribozymes in a CML cell line, K562. We demonstrated a 53% decrease in bcr-abl mRNA levels in a clone induced to express Rz8, compared with its uninduced control. Phenotypic analysis of this clone also revealed a 63% decrease in colony-forming ability and a 43% inhibition of cell proliferation following ribozyme expression. Morphologic analysis of cells showed there was a slight increase (2.5% to 15%) in the number of cells undergoing apoptosis. These results suggest that Rz8 was effective in suppressing bcr-abl gene expression within a cellular environment and altering the leukemic nature of a CML cell line.