Regression of lymphoma xenografts after PRIT comparing anti-CD20 B9E9 (scFv)₄SA fusion protein and 1F5 Ab-SA chemical conjugate. Athymic BALB/c mice bearing Ramos lymphoma xenografts were injected intraperitoneally with either 1.4 nM B9E9 (scFv)₄SA (•) or 1F5 Ab-SA (▵), followed 20 hours later by 5.8 nM CA, and 4 hours after that with (A) 200, (B) 400, (C) 800, (D) 1000, or (E) 1200 μCi (7.4, 14.8, 29.6, 37, or 44.4 MBq, respectively) 90Y-DOTA-biotin. Tumor volume curves are truncated at the time of euthanasia of the first mouse in each group. Control mice bearing xenograft tumors were treated with pretargeted CC49 (scFv)₄SA (▪).