A method of preparing simultaneously monoclonal antibodies specific for different cell-free products of activated human T lymphocytes is disclosed. Human T cells are activated in a medium supplemented with mouse serum rather than conventional calf serum. A supernatant prepared from the activated T cells is used to immunize mice. The dominant immunogens in the supernatant are the cell-free products of human T lymphocytes. The yield of hybrid cells which produce products reactive with cell-free products of human T lymphocytes is enhanced by injecting the immunized mice with a supernatant from mitogen-activated murine splenocytes. In addition, a novel radioimmunoadsorbent assay for screening hybrids to detect production of monoclonal antibodies reactive with cell-free products of human T lymphocytes is disclosed.