DIFFERENCES IN IMMUNIZATION AND SENSITIZATION IN RABBITS INJECTED WITH RELATIVELY AVIRULENT OR HIGHLY VIRULENT CULTURES OF THE SAME STRAIN (H) OF HEMOLYTIC STREPTOCOCCUS.

1. When a relatively avirulent strain of hemolytic streptococcus was injected into the skin of rabbits a considerable amount of skin sensitivity to streptococcus filtrate developed, and there was scant demonstrable immunity. 2. When the same strain of streptococcus was made highly virulent by repeated passages through rabbits, little sensitization and considerable immunity was produced. 3. Rabbits injected with a virulent culture of hemolytic streptococcus developed agglutinins and precipitins earlier and in larger amounts than animals injected with the avirulent form of the same organism. 4. Increase in virulence of hemolytic streptococci enhances the ability to protect against local infection and increases antibody formation but diminishes the production of sensitization. Avirulent and virulent strains of this microorganism have similar relations.