Pretargeting methods and compounds

An improved method of diagnosis or therapy through the use of a polyalkylene glycolated streptavidin anti-ligand whose biotin-binding sites have been protected prior to polyalkylene glycolation, which method includes the administration of a conjugate containing an antibody, or antigen-binding antibody fragment attached to a ligand or antiligand and the concurrent or subsequent administration of a second conjugate containing a ligand or anti-ligand which binds the ligand or anti-ligand contained in the first conjugate, and which is attached either directly or indirectly to a radionuclide active agent, wherein the improvement comprises the attachment of one or more polyalkylene glycol residues to at least the anti-ligand, wherein said antiligand is streptavidin and optionally further the targeting moiety or active moiety which is contained in the first or second conjugate, and wherein such attachment is effected after the biotin-binding sites contained in the streptavidin have been protected by a molecule which reversibly binds to biotin binding sites.