1. Pneumococci, dysentery bacilli of the Shiga type, and Bacillus mucosus capsulatus are agglutinated immediately when injected into the circulation of actively immunized rabbits. 2. Staphylococcus aureus and albus, colon bacilli, meningococci, gonococci, and non-virulent pneumococci agglutinate in the circulation of normal rabbits. 3. Bouillon cultures of Bacillus avisepticus are highly toxic for both rabbits and dogs. The fresh sera of these animals have no bactericidal action upon the bacteria. Dog serum opsonized the bacilli in vitro, and they are agglutinated and opsonized in the circulation and organs of normal dogs. On the other hand, none of this occurs in connection with normal rabbits. A very small quantity of culture produces a fatal septicemia in rabbits, but a subtoxic dose is without effect in dogs. 4. The degree of agglutination and opsonization of bacteria within the animal body is inversely parallel to the infectiousness of the bacteria for the host.