We present evidence that the hereditable hemolytic disease, hereditary spherocytosis (HS), involves an abnormality in protein of the red cell membrane. Unlike that from normal red cells, lipid-free proteins extracted from HS red cell membranes fail to increase in sedimentation rate when treated with cations; such treatment of normal membrane proteins has been shown by others to cause the formation of microfilaments. That microfilament formation might be defective in HS red cell membranes is supported by observations with vinblastine. This compound, a potent precipitant of filamentous, structure proteins throughout phylogeny, precipitates significantly less HS membrane protein than normal. The resistance of HS membrane protein to changes in conformation by cations is observable at the cellular level as well. That is, both normal and HS red cells agglutinate after repeated washing and suspension in electrolyte-free media. Tiny concentrations of Ca(++) (5 x 10(-5) M) changes the surfaces of normal cells in such a way as to cause disagglutination; HS red cells resist this change and remain agglutinated unless Ca(++) concentrations are increased many-fold. We conclude that membrane ("structure") proteins of HS red cells are genetically altered in such a way as to interfere with their proper conformation, perhaps into fibrils. Potentially many mutations in membrane proteins might preclude this alignment, with the result that normal erythrocyte biconcavity and plasticity is prevented and the clinical syndrome of hereditary spherocytosis is manifest.