Human T cell Lymphotropic Virus-I (HTLV-I) carrying human T cell line MT4 is highly sensitive to Human Immunodeficiency Virus-1 (HIV-1). After HIV-1 infection cell clusters characteristic of intact MT4 rapidly disintegrate, syncytia appear and the cells die. Surviving MT4 cells were subcultured following HIV-1 infection of high multiplicity. We succeeded to establish an MT4 cell line continuously producing infective HIV (MT4/HIV-1). The original and the HIV-1 infected MT4 cells were morphologically similar. The MT4/HIV-1 cells proved to be nearly 100% positive in indirect immunofluorescence assay using the serum of an HIV-1 antibody positive individual. OKT4 surface antigen could not be demonstrated on MT4/HIV-1 cells. On electron microscopic pictures typical and atypical virus particles could be seen near the surface of the cell membrane. The persistently produced virus particles were infective for H9 and MT4 cells. The antigenic structure of the virus produced by MT4 cells was similar to that produced by H9 cells.