Serum levels of immunosuppressive acidic protein (IAP) in 105 patients with hematopoietic malignancies, there were 12 cases of acute myeloblastic leukemia, 1 acute monocytic leukemia, 13 myelomonocytic leukemia, 4 acute promyelocytic leukemia, 26 chronic myelogenous leukemia, 22 non-Hodgkin's lymphoma, 5 Hodgkin's disease, 6 adult T-cell leukemia, 5 acute lymphoblastic leukemia, 3 chronic lymphocytic leukemia, and 8 multiple myeloma. High levels of serum IAP were detected in all of the patients except chronic phase of CML, malignant lymphoma in stage I and II, and multiple myeloma. In the cases of malignant lymphoma, serum IAP levels in stage III and IV were higher with statistical significance (p less than 0.01) than those in stage I and II. Serum IAP levels in the patients with CML in blastic crisis were higher than in the chronic phase, so serum IAP levels are useful as one diagnostic parameters in blastic crisis. However, in patients with ANLL in relapse, serum IAP levels showed normal values. Serum IAP levels paralleled those of acute phase reactants such as alpha 1-acid glycoprotein , C-reactive protein, alpha 2-globulin, and alpha 1-antitrypsin, and had inverse correlations with PPD and PHA skin test.