Biochemical properties of mouse antimouse CD28 mAb E18. (A) Anti-CD28 mAb E18 blocks binding of B7 to CD28. Mouse thymocytes were incubated with E18, E18 Fab, or mAb 37.51 as reference before binding of mouse CD80-human Ig (mCD80huIg) was assessed. (B) Titration of E18, its Fab fragment, and 37.51 (C) in assays analyzing ligand blocking activity as in panel A.