The reciprocal translocation, which constitutes the Philadelphia (Ph) chromosome (t(9;22)(q34;q11) ), characterises more than 90% of patients with chronic myeloid leukaemia (CML). We report here that the Ph chromosome and/or the bcr-abl fusion in CML cases were determined by using the conventional cytogenetic analysis, fluorescence in situ hybridisation (FISH) with locus specific (unique sequence) cosmid probes for abl and bcr genes, and with painting probes for chromosomes 9 and 22, and reverse transcriptase - polymerase chain reaction (RT-PCR) techniques. Cytogenetic trypsin G - banding analysis was found to be broadly in agreement with the results of FISH, and metaphase-FISH is a suitable method for quantifying the proportion of abnormal cells. In addition, the break points of the abl and the bcr genes were demonstrated by RT-PCR technique. Thus, the results illustrate that each technique has a potential usefulness for detecting abnormalities over others.