The detection of the t(9;22) translocation, typical of chronic myeloid leukaemia (CML), can be accomplished by cytogenetical detection of Philadelphia (Ph1) chromosome or by molecular analysis of the bcr/abl fusion gene with nucleic acid probes after amplification by polymerase chain reaction (PCR). PCR-based approaches are now widely used for follow up of CML patients during therapy or after bone marrow transplantation (BMT). We describe here a microtitre, colorimetric assay (DNA Enzyme Immunoassay, DEIA) for analysis of t(9;22) translocation after enzymatical amplification of RNA from CML patients. This assay is based on the use of a monoclonal antibody specifically reacting with double stranded DNA, i.e. with hybridized DNA. The assay represents a nonisotopic alternative to other current hybridization assays and requires no modifications of primers, probe or target DNA.