Acute promyelocytic leukaemia (APL) is characterized cytogenetically by t(15;17)(q22:q21) which results in the production of a PML/RAR alpha fusion protein. Detection of the translocation or the fusion gene product is required for objective diagnosis of APL. This can be accomplished by conventional cytogenetic methods, fluorescence in situ hybridization or RT-PCR. Such techniques are time consuming and not universally available. The intracellular distribution of the PML protein in promyelocytes is characteristically altered in APL and this can be detected by immunocytochemistry. We have assessed two immunocytochemical methods, immunofluorescence and alkaline phosphatase-anti-alkaline phosphatase staining (APAAP), with regard to sensitivity, specificity and rapidity of diagnosis. 85 patients with AML including 15 cases of APL were studied. Immunofluorescence PML detection was concordant with RT-PCR for t(15:17) in 14/15 (93.3%) cases with no false positives. The negative APL case in our series was a patient with a 5' PML breakpoint who did not express the reciprocal t(17;15) fusion product. APAAP was concordant in only 6/13 (46%) APL cases with one false positive. In conclusion, immunofluorescent localization of PML using 5E10 monoclonal antibody is a rapid, sensitive and specific diagnostic tool for APL.