A Technicon H-1 hematologic analyzer was used to measure the mean leukocyte myeloperoxidase (MPX) in 160 patients seen in a hematology clinic. The normal range was -15 to +10, which included 95% of 300 consecutive hospitalized patients. No abnormalities in the MPX were found in 35 patients with beta-thalassemia minor, 8 with iron deficiency, 14 with myeloproliferative disorders, 17 with autoimmune disorders, and 37 patients with lymphoma in complete remission. On the other hand 36% (10/28) of lymphoma patients with active disease either at diagnosis or relapse had a MPX of greater than 10 compared to only 2.3% (7/300) in hospitalized patients (P less than 0.001). Increased levels of MPX were found primarily in patients with non-Hodgkin's lymphoma (NHL) of intermediate or high grades, or Hodgkin's disease [56% (9/16) compared to only 8.3% (1/12) in those with low grade NHLs, P less than 0.05]. The MPX levels returned to normal after successful treatment. Of the various chemotherapeutic agents used, only hydroxyurea led to a consistent elevation of the MPX. The authors conclude that MPX is commonly increased in patients with lymphoma and in those receiving hydroxyurea. Further studies are required to determine if the MPX is a sensitive test for relapse in patients with lymphomas who had an elevated pretreatment value.