An enzyme immunoassay (EIA) was developed for detection of Human T-cell Leukemia Virus antigen in culture supernatants and cell lysates. The assay used a mouse monoclonal antibody against HTLV-I p19 major core protein as capture antibody. It has a sensitivity of 1 microgram/ml of HTLV-I protein, 250 pg/ml of purified recombinant p19 and detected p19 in an 10(-2) diluted supernatant of MT2 infected cell and in a 100 MT2 cells lysate (10(6) cells taken at day 7 of culture). The assay enable us to discriminate between HTLV-I and HTLV-II antigens and is reproducibly negative for supernatants and cell lysates of uninfected cells and of HIV-1 infected cells. The assay was found to be more specific and 10 times more sensitive than the reverse transcriptase (RT) assay, and the EIA test became positive three days earlier than RT assay for the HTLV-I cell lines supernatants.