1. The titer of an individual insulin antiserum measured by hemagglutination agrees favorably with the titer of the same antiserum as measured by immune hemolysis.
2. In contrast, a marked decrease is noted in the immune hemolysis titers relative to hemagglutination titers of antisera prepared against three different sialic acid-containing proteins (human chorionic gonadotropin, rabbit transferrin, and human transferrin).
3. The lower immune hemolytic titers of glycoprotein antisera are apparently not due to a lack of complement-fixing γ2-antibodies.
4. The glycoprotein antigens in solution do not interfere with hemolysis in the insulin immune system.
5. By contrast, marked inhibition of insulin immune hemolysis occurs when cells are sensitized with both glycoprotein and insulin.
6. Cells treated with neuraminidase (to remove cell surface sialic acid) lyse in the presence of C'a alone.
7. If neuraminidase-treated cells are sensitized with sialic acid-containing protein, the lysis of these cells by complement is inhibited.
8. It is, therefore, postulated that during some initial phase of immune hemolysis, a sialic acid-containing substrate is cleaved from the cell surface, rendering the cell susceptible to lysis. If this removal is interfered with, i.e., by sensitization of the cell with competitive sialic acid-containing antigen, then the lytic portion of immune hemolysis cannot proceed.