We performed immunocytochemical detection of myeloperoxidase (MPO), using monoclonal antibody MPO-7, in 15 consecutive cases of adult acute leukemia (AL) unclassified by conventional cytological and cytochemical criteria and 7 AML-M1 with less than 10% of cytochemically MPO-positive blasts. In AL with negative MPO cytochemistry the anti-MPO reaction was positive in 5 of the 15 patients with 3, 3, 7, 11 and 45% positive blasts respectively. In AML-M1, immunocytochemistry was positive in a larger percentage of blasts than cytochemistry in 2 cases. Immunological detection of myeloid surface markers was positive in all 15 cases of unclassified AL (including the 10 AL with negative anti-MPO reaction). Eleven of the 22 patients from this study had mixed lymphoid-myeloid phenotype. Discrepancy between immunological MPO detection and light cytochemistry was more frequent in patients with mixed immunophenotype than in patients without lymphoid markers. No relationship between MPO-antigen positivity and clinical or biological features was seen. These findings confirm immunological detection of MPO as useful for the diagnosis of poorly differentiated AL. The high incidence of inactive MPO detectable only by immunocytochemistry in mixed lineage AL needs to be confirmed.