Abstract
Mg++ was removed from guinea pig and human sera by diaiyzing the serum in the presence of an ion-exchange resin. Complete or almost complete removal of Mg++ from reagents used in the sheep cell hemolytic system, with the concn. of Ca++ being maintained, diminished the complement titer of guinea pig serum by about 25%. Antibody fixation was only slightly reduced when Mg++ was lacking in the cold phase of the Donath-Landsteiner reaction. Max. hemolysis was obtained when Mg++ was present in both the cold and warm phases. Mg++ does not appear to be essential for hemolysis in the Donath-Landsteiner reaction of paroxysmal cold hemoglobinuria.