We examined the susceptibility of a variety of human leukemic cell lines to the induction of apoptosis. K562, a chronic myelogenous leukemic cell line which expresses the bcr-abl fusion gene, was found to be extremely resistant to apoptosis, irrespective of the inducing agent. This resistance can be attributed to the deregulated Abl kinase activity of bcr-abl, as downregulation of its expression using antisense oligodeoxynucleotides targeted to the beginning of the abl sequence in this chimeric gene rendered these cells susceptible to cytotoxic drug-induced apoptosis. Examination of the morphological and biochemical features of apoptosis in K562 cells revealed the typical membrane blebbing and chromatin condensation associated with this form of cell death. In situ TdT-mediated end labeling of the DNA revealed the presence of strand breaks in the treated cells and field inversion gel electrophoresis revealed the presence of large 10-50 kb fragments. However there was an absence of oligonucleosomal DNA fragmentation, whether or not Bcr-Abl was expressed. Thus, while inhibition of expression of Bcr-Abl renders K562 cells susceptible to apoptosis, the absence of oligonucleosomal DNA fragmentation in these cells is independent of the function of this molecule.