Abstract: The prevent invention relates to two monoclonal antibodies which are broadly reactive with all normal human peripheral blood T and B lymphocytes and monocytes and to hybridoma cell lines which produce these monoclonal antibodies. These monoclonal antibodies are designated WM-66 and WM-63 and the respective hybridoma cell lines are designated F56-2D7 (ECAC 88101104) and F56-4D6 (ECACC 88101103). Both of these monoclonal antibodies react with a previously unrecognized human leucocyte differentiation antigen. The relative molecular mass of the antigen recognized by WM-66 is approximately 65 kilodaltons.