Fragment I was isolated from rabbit γ-globulin which contained antibodies to Salmonella. On a weight basis the fragment I and γ-globulin were compared as to their ability to promote phagocytosis, protect mice against infection with Salmonella typhimurium, and prepare bacteria for the bactericidal effect of complement. Fragment I retains 1 to 2% of the activity of intact γ-globulin in clearance and protection tests. It retains no activity in the bactericidal test nor in the sensitization of red cells for complement lysis. Although certain components of complement may be involved, the complete system as defined by hemolytic activity or bactericidal activity is not essential either for opsonization of Salmonella or for preparation for in vivo kill.

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