1. An artificial antigen containing the azobenzyl glycoside of cellobiuronic acid gives rise in rabbits to antibodies which: (a) give the Neufeld reaction and agglutinate Type III pneumococci, (b) confer passive protection on mice against infection with Types II, III, and VIII pneumococci. 2. Rabbits immunized with the artificial cellobiuronic acid antigen acquire active resistance to infection with virulent Type III pneumococci. 3. The antibodies evoked by an antigen containing the azobenzyl glycoside of cellobiose exhibit none of these phenomena.