When rabbits were fed S35-labelled amino acids and simultaneously injected with antigen at the peak of antibody production, the circulating antibody became rapidly labelled within a few hours with the isotope. The specific radioactivity of antibody was measured as antibody was allowed to decline in the absence of antigenic stimulation. At various times, in different animals, antigen was reinjected and circulating antibody was measured for specific radioactivity. The initial antibody which appeared after the antigenic stimulus always had a higher specific activity than antibody circulating just prior to the reinjection. The appearance of antibody of higher specific activity was demonstrated to be a specific response to the antigen which was reinjected. It was concluded that there is a reservoir of antibody which is stabilized in tissue and which is not in equilibrium with that in circulation. A mechanism for this stabilization is suggested and discussed from previous investigations demonstrating the long retention of antigen in liver tissue.

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