Material that binds B7.1 and B7.2 can only be isolated from blood that contains molecules recognized by CTLA-4 specific antibodies. Plasma from subjects 17 (CTLA-4 negative) and 19 (CTLA-4 positive) were passed over a column linked to the CTLA-4 specific antibody AS32. Material that eluted from the column under stringent wash conditions was collected in fractions, and was used to coat the wells of a 96 well plate. In panel (A) these wells were probed with the CTLA-4 specific antibody AS33. Panel (B) shows the binding of B7.1-Ig, B7.2-Ig, and Muc-Ig to the coated wells.