We have previously shown, using human T-cell lymphocytotrophic virus-I (HTLV-I)-infected cell lines, that soluble interleukin-6 receptor (sIL-6R) is generated through an alternative splicing mechanism. In this study, we examined human sera for the presence of alternatively spliced soluble IL-6R (AS-sIL-6R). We produced a monoclonal antibody (mAb) recognizing the unique sequence of AS-sIL-6R peptide, generated by an altered reading frame. We also made recombinant AS-sIL-6R protein in Spodoptera frugiperda-9 (Sf-9) cells carrying baculovirus, which encoded altered sIL-6R or conventional IL-6R cDNA. mAbs specifically recognized AS-sIL-6R, but not conventional IL-6R, as demonstrated by Western blot analyses, fluorescence-activated cell sorter, immunofluorescence analyses and enzyme-linked immunosorbent assay (ELISA). We adapted an ELISA system and used it for detection of altered sIL-6R in sera from 23 healthy persons, 12 patients with adult T-cell leukaemia (ATL) and 13 patients with HTLV-I-associated myelopathy (HAM). Serum levels of AS-sIL-6R were 6.4 or 6.1 times greater in ATL (28.7±20.4 ng/ml, P<0.0001) and in HAM patients (27.5±12.1 ng/ml, P<0.0001) than in healthy individuals (4.5±2.1 ng/ml). High levels of AS-sIL-6R were also observed in plasma from rheumatoid arthritis patients and in persons with elevated levels of alanine aminotransferase (ALT), antinuclear antibody (ANA), or α-fetoprotein (AFP). However, in human immunodeficiency virus-1 (HIV-1), hepatitis B virus (HBV) or hepatitis C virus (HCV)-infected individuals, AS-sIL-6R levels were not elevated. In this study, we confirmed that AS-sIL-6R is indeed present in human sera. These observations suggest that alternative splicing of IL-6R mRNA is of consequence in ATL, HAM and in some autoimmune diseases. The HTLV-I-infected T cells appeared to play an important role in AS-sIL-6R production.