A radioimmunoconjugate comprised of a recombinant scFv dimer diabody of a monoclonal antibody against human carcinoembryonic antigen (CEA) labelled with iodine I 123 (I-123) with potential radioimmunolocalization applications. The antibody moiety of iodine I 123 anti-CEA recombinant diabody T84. 66 binds to cells expressing CEA, selectively delivering I-123 upon cellular internalisation and allowing the scintigraphic imaging of CEA-expressing tumour cells. CEA, a tumour associated antigen, is overexpressed in many cancer types, including gastrointestinal, breast, non-small cell lung, and thyroid cancers. Compared to whole monoclonal antibodies, diabody fragments offer the advantages of rapid tumour targeting, rapid blood clearance, more uniform tumour distribution, and a lower potential for eliciting an immune response.