Disclosed are methods relating to cell membrane fragments associated with microbeads, so that the characteristics of the cells the fragments originated from can be determined. The fragments can be oriented with what was the outer surface of the cell membrane facing outwardly, so that the antigens associated with the membrane can be contacted with ligands (including antibodies) to antigens in the membranes which would be accessible to antibodies in vivo. The system is useful, inter alia, for detection of panel reactive antibodies in donor serum, as well as detection of other cell membrane antigens; or quantitation of particular cell membrane antigens.