Western blot analysis of the specific reactivity of rituximab with phage minor-coat PIII protein-fused peptides (arrows) from the positive ELISA phage clones. Purified phage particles (1 × 10¹⁰ pfu's/lane) isolated by biopanning rituximab with c7c (A,C) and 7- or 12-mer (B,D) PDPLs were run onto a 10% polyacrylamide gel under nonreducing conditions and transferred to a PVDF filter. The filter was incubated with biotinylated rituximab (A-B) and the assay was continued as described in “Affinity selection, immunoscreening, Western blot, and sequence analysis.” Binding of rituximab to anti-CD4 mAb HP2/6-specific phage clone (HP41-C) and of biotinylated mAb HP2/6 with rituximab-specific phage particles (C-D) were included as specificity controls. MW indicates molecular weight.