The increase in the serum levels of the IL-2 receptors is due to its release both in vivo and in vitro from activated cells or neoplastic cells expressing it constitutively. The diagnostic, prognostic and physiopathologic significance of the sIL-2R was investigated by testing the serum of 271 haemopathic patients in various stages of the disease. In HCL the elevated sIL-2R level has a diagnostic value. In HD the sIL-2R level appears to be directly correlated with the extent of the disease and is equally important in the follow up of patients with HCL, NHL, HD, AL and MDS, where the serum level of the soluble receptor is usually associated with the biological and clinical activity of the disease. Unlike other B lymphoproliferations, patients with Multiple Myeloma on average show only slightly elevated levels of soluble receptor with no significant differences related to the stage or evolution. As for the chronic myeloproliferative disorders, we found only slightly elevated values in ET and PV, with frankly pathological values in CML during a blastic crisis or in the accelerated phase and in MFI during the clinically active phase of the disease.