The proliferative response of human blood lymphocytes to homologous cells in vitro (i.e. the mixed lymphocyte reaction) has been used as a model of cell-mediated immune responses to investigate the nature of the antigen receptor site involved in the initiation of these reactions. Pretreatment of lymphocytes with the Fabγ monomer of rabbit antisera to human immunoglobulin light chain determinants effectively suppressed subsequent responsiveness to antigen without impairing lymphocyte viability or response to phytohaemagglutinin. Experiments in which the dose of antigen and the time of addition of antigen relative to antiserum were varied suggested that anti-light chain antibodies compete with antigen for the receptor site. These results are discussed in relation to the problem of the chemical nature of the receptor site for transplantation reactions and delayed hypersensitivity.