This invention provides: a heteromyeloma, other than B6B11, capable of producing a trioma when fused with a human lymphoid cell, wherein the trioma is capable of producing a monoclonal antibody-secreting tetroma when fused with a second, antibody-secreting human lymphoid cell; a trioma fusion partner which does not produce antibody, obtained by fusing a heteromyeloma which does not produce antibody with a human lymphoid cell; a monoclonal antibody-secreting tetroma, obtained by fusing a trioma which does not produce antibody with an antibody-secreting human lymphoid cell; a method of producing a monoclonal antibody that specifically recognizes an antigen associated with a condition; a method of identifying an antigen associated with a condition using the trioma fusion partner; a method of diagnosing a condition using the trioma fusion partner; a method for preventing a condition; and compositions and therapeutic compositions comprising monoclonal antibodies produced using the trioma fusion partner.