Abstract: A hybrid cell line is provided which is capable of producing monoclonal antibodies which bind to HIV core antigen P55, the precursor protein coded for the gag gene, the core protein P24 and partial breakdown products P39 and P33. The monoclonal antibody embodying the invention does not bind the P18 core protein or any HIV envelope antigens. The cell line of the invention was developed by a unique immunization protocol in which BALB/c mice were immunized over a series of multiple infusions using a select group of immunogens and a conventional myeloma cell line for fusion with the murine splenocytes harvested.The monoclonal antibody is identified as the KC-57 antibody. This monoclonal antibody is especially useful for a solid phase immunoassay in which the monoclonal antibody is detected in its binding to HIV antigens found in a serum or plasma sample from a human patient. The cell line which produces the KC-57 monoclonal antibody has been deposited in the American Type Culture Collection, Rockville, Md.