The 12- to 14-kDa eosinophil major basic protein (MBP) is primarily translated as 25-kDa pro-MBP. HL-60, a promyelocytic leukemia cell line, produces pro-MBP but not MBP, suggesting production of pro-MBP by immature granulocytes. We measured the serum levels of total MBP, using an ELISA that detects both pro-MBP and MBP, in 25 patients with leukemia (six acute myelogenous leukemia (AML), seven acute lymphoblastic leukemia (ALL), eight chronic myelogenous leukemia (CML), four chronic lymphocytic leukemia (CLL)) and five recipients of allogeneic bone marrow transplants (BMT). None of these patients except one AML (M4Eo) showed eosinophilia. Serum levels of total MBP were elevated in all the patients with CML and AML, but not in any of those with ALL or CLL. In all four recipients of BMT who obtained engraftments, serum levels of total MBP started to increase 12-14 days after BMT and reached the highest levels (4-10 times the basal levels) at days 19-32. In a recipient of BMT who did not obtain an engraftment, serum levels of total MBP were not increased. These findings suggested that pro-MBP could be used as a marker for proliferation of immature granulocytes with normal or malignant nature.