Endotoxin subunits normally aggregate into vesicles with a molecular mass around 1,000,000. Some proteins can bind and disaggregate endotoxin to form protein-endotoxin complexes, which makes endotoxin removal from protein solution more complicated. It is also known that divalent cations such as Ca2+ can act as "bridges" between lipopolysaccharide subunits, causing large vesicles to form. Such a property has potential utility in endotoxin removal from biological solutions. A two-stage ultrafiltration was used in this study during which Ca2+ reaggregated lipopolysaccharide subunits in protein solutions into large vesicles that hence were retained by a 300,000 nominal molecular weight cutoff ultrafiltration membrane. After the reaggregation of lipopolysaccharide subunits, the endotoxin removal efficiency was in the range of 54.78 to 73.10%, meanwhile protein yield was in the range of 71.57 to 89.54% at various protein/endotoxin concentration ratios.