FACS analysis of human CR1 in intact versus permeabilized RBL-CR1 cells. Cells were fixed in periodate/lysine/paraformaldehyde with (B) and without (A) permeabilization with saponin. Cells were then stained with anti-CR1 monoclonal antibody 3D9 or control antibody MOPC 21; followed by FITC-conjugated antimouse antibody. A total of 10,000 cells from each preparation was analyzed. Mean fluorescence values are: (A) background (gray line) 66; CR1 (black line) 99. (B) Background (gray line) 109; CR1 (black line) 232.