Résumé : Three major types of mRNA can be expressed as a result of the Philadelphia translocation, dependent on the position of the break within the BCR gene on chromosome 22. In addition, alternative splicing of the mRNA transcribed from the BCR/ABL fusion gene has been reported and it has been suggested that this may play a role in the generation of the acute phase of Philadelphia positive chronic myeloid leukaemia (CML). We have examined the fusion RNA present in 24 cases of chronic phase CML and 21 cases of patients with CML in blast crisis using the polymerase chain reaction. In no case was it possible to detect the presence of the e1a2 junction which encodes the p190 hybrid protein product. We conclude that the acquisition of the p190 does not play a significant role in the generation of the blast crisis of CML. Neither could we detect a significant difference in the number of cases which simultaneously express both b2a2 and b3a2 junction products in samples isolated from chronic phase and blast crisis. In the series analysed by ethidium bromide stained gels, there was, however, an increase in the percentage of cases expressing the b3a2 junction in the mononuclear cells of blast crisis patients as compared to the white blood cells of patients in chronic phase.