INTRODUCTION: Myeloperoxidase (MPO) is present in azurophilic granules which appear in the promyelocyte stage of differentiation and is expressed in granulomonocytic cells. MPO is usually detected by cytochemistry. The demonstration of peroxidase in at least 3% of bone marrow blasts defines an acute leukaemia as acute myeloblastic leukaemia (AML). MPO is important in distinguishing acute myeloblastic leukaemia (AML) from acute lymphoblastic leukaemia (ALL). It is difficult to diagnose AML with minimal evidence of myeloid differentiation (AML- M0) by conventional light microscopy. However, these AML-M0 blasts can be detected by monoclonal antibodies. Anti-MPO recognizes the enzymatically inactive precursor forms of MPO. There are a few commercially available monoclonal antibodies against MPO. In this study, we evaluated two monoclonal antibodies against MPO from different commercial sources. METHODS: Anti-MPO were purchased from Dako (Denmark) and Becton Dickinson, BD (California, USA). MPO detection was done using the permeabilisation-staining technique, followed by analysis with flow cytometer (FASCalibur, California, USA). RESULTS: 63 cases of acute leukaemias (38 ALL and 25 AML) were studied. Anti-MPO by Dako showed that 12/38 (31.6%) of ALL cases were positive, but all these cases were clear-cut negative for anti-MPO from BD. 24/38 (63.2%) of these ALL cases were associated with aberrant expression of myeloid antigens. However, only 8/24 (33.3%) cases with aberrant myeloid antigen expression showed positive reaction to anti-MPO (Dako). 23/25 (92%) of AML showed concordance results for both anti-MPO by Dako and BD. CONCLUSION: Anti-MPO is a useful and reliable marker for the diagnosis of AML. However, this study had demonstrated that results vary with the monoclonal antibody used in ALL cases. Anti-MPO (Dako) had shown false positive result in 31.6% of ALL cases whereas anti-MPO (BD) had shown consistent negative result in ALL cases.