Recent reports have shown that HTLV I infection is widespread and that HIV/HTLV I combined infections may accelerate the progression of seropositive patients towards AIDS. In order to obtain a tool for the detection and identification of HTLV I in cell culture we have developed an antigen-capture ELISA, which is based on the use of rabbit hyperimmune serum on solid phase and mouse monoclonal antibodies against p19 and p24 as tracers. This assay has a sensitivity of 50 ng/ml, it can be used to test cell-culture fluids and does not cross-react with any other known human retroviruses.