We have established an interleukin 2-dependent, OKT4-positive T-cell line, named HCT-1, from cells in the cerebrospinal fluid obtained from a patient with human T-cell lymphotropic virus type I (HTLV-1)-associated myelopathy. Antigens for HTLV-I were detected in HCT-1 cells by indirect immunofluorescence and Western blot testing, and type C virus particles were detected by electron microscopy. Southern blot analysis of HCT-1 cellular DNA, using an HTLV-I probe, revealed that the integrated provirus genome could not be distinguished from the HTLV-I genome in adult patients with T-cell leukemia.