The determinants of cell shape were explored in a study of the crenation (spiculation) of the isolated erythrocyte membrane. Standard ghosts prepared in 5 mM NaPi (pH 8) were plump, dimpled disks even when prepared from echinocytic (spiculated) red cells. These ghosts became crenated in the presence of isotonic saline, millimolar levels of divalent cations, 1 mM 2,4-dinitrophenol or 0.1 mM lysolecithin. Crenation was suppressed in ghosts generated under conditions of minimal osmotic stress, in ghosts from red cells partially depleted of cholesterol, and, paradoxically, in ghosts from red cells crenated by lysolecithin. The susceptibility of ghosts to crenation was lost with time; this process was potentiated by elevated temperature, low ionic strength, and traces of detergents or chlorpromazine. In that ghost shape was influenced by a variety of amphipaths, our results favor the premise that the bilayer and not the subjacent protein reticulum drives ghost crenation. The data also suggest that vigorous osmotic hemolysis induces a redistribution of lipids between the two leaflets of the bilayer which affects membrane contour through a bilayer couple mechanism. Subsequent relaxation of that metastable distribution could account for the observed loss of crenatability.