An immunoconjugate containing a monoclonal antibody directed against the CD45 antigen BC8, conjugated to streptavidin, a nonglycosylated homotetrameric protein that has four high affinity binding sites for biotin. Anti-CD45 BC8 antibody-streptavidin conjugate binds to CD45, a transmembrane protein tyrosine phosphatase that is expressed on the surface of normal and malignant hematopoietic cells. Upon administration of a biotin-based radioconjugate, the biotin moiety of the radioconjugate binds to the streptavidin moiety of anti-CD45 BC8 antibody-streptavidin conjugate and, upon cellular internalization, specifically delivers cytotoxic radiation to CD45-expressing tumor cells.