Myeloperoxidase is a component of the microbicidal network of polymorphonuclear leukocytes. The enzyme is a tetramer consisting of two heavy and two light subunits. A large proportion of humans demonstrate genetic deficiences in the production of myeloperoxidase. As a first step in analyzing these deficiencies 1n more detail, we have isolated cDNA clones for myeloperoxidase from an expression library of the HL-60 human promyelocytic leukemia cell line.Two overlapping plasmids(pMP02 and pMP062) were identified as myeloperoxidase cDNA clones based on the detection with myeloperoxidase antiserum of 1)70 kDa protein expressed 1n pMP02-conta1n1ng bacteria and 2) a 75 kDa polypeptide produced by hybridization selection and translation using pMP062 and HL-60 RNA. Formal identification of the clones was made by matching the predicted amino add sequences with the amino terminal sequences of the heavy and light sub-units. Both subunits are encoded by one mRNA in the following order: pre-pro-sequences -- light subunit -- heavy subunit.The molecular weight of the predicted primary translation product 1s 83.7 kDa. Northern blots reveal two size classes of hybridizing RNAs(approximately 3.0–3.3 and 3.5–4.0 kilobases) whose expression is restricted to cells of the granulocytic lineage and parallels the changes in enzymatic activity observed during differentiation.