PURPOSE: To provide the titled receptor capable of determining the membrane- bonded tumor antigen and free tumor antigen qualitatively or quantitatively, easily in high sensitivity, by bonding a labeling agent to a T-cell antigen receptor capable of specifically bonding to an antigen.

CONSTITUTION: A T-cell antigen receptor derived from a broad tumoricidal killer T-cell clone is bonded with a labeling agent (e.g. fluorescein isothiocyanate or rhodamine which are fluorescent pigments, radioactive 125I, etc.) to obtain the titled receptor. The tumor antigen on a tumor cell membrane can be identified by adding the receptor to the tumor cell. Furthermore, free tumor antigen can be determined quantitatively and easily in high sensitivity by radioimmunoassay. It can be applied effectively to the qualitative and quantitative determination of virus antigen, bacterial antigen, histocompatibility antigen, etc., in addition to the tumor antigen.