Rituximab pretreatment impairs anti-CD20 tumor targeting, but not anti-CD45 targeting in Ramos xenografts. Effect of rituximab pretreatment on percent injected radioactivity dose per gram (%ID/g) of normal and tumor tissue in Ramos xenograft-bearing mice treated with (A) ¹²⁵I-BC8 and (B) ¹³¹I-tositumomab. Tumor-to–normal organ ratios of delivered radioactivity after pretreatment with rituximab or 1D8 followed by (C) ¹²⁵I-BC8 or (D) ¹³¹I-tositumomab. Error bars represent SE.