Abstract
When fixed erythrocytes are exposed to the cholesterol probe, filipin, and freeze-fractured, their membrane is labeled with only a limited number of typical, 25 nm diameter filipin-sterol complexes. In addition, the membrane contains many less distinct filipin-induced perturbations that give it an overall rippled appearance. In contrast, red cell ghosts subjected to conditions that result in an aggregation of their intramembrane particles have large particle-free membrane domains which are completely filled with 25 nm filipin-sterol complexes. These results suggest that some constraint exists in the intact erythrocyte membrane which restricts the formation of typical filipin-sterol complexes.