A radioimmunoconjugate consisting of the murine IgG1 monoclonal antibody BW250/183 labeled with yttrium Y 90 with potential radioimmunotherapeutic activity. Yttrium Y 90 anti-CD66 monoclonal antibody BW 250/183 binds to a 95 kD nonspecific cross-reacting antigen (CD66b or NCA-95) on granulocytes, selectively delivering a cytotoxic dose of Y 90 beta radiation to CD66b-expressing cells. CD66b, a member of the carcinoembryonic antigen (CEA) family with an unknown function, appears early in the differentiation of granulopoietic cells and is expressed on the cell surface of almost all human granulocytes and their more mature precursors.