1. The half-lives of homologous gamma globulins labelled with I131 agree well with the half-lives of passively administered homologous antibodies and provide a simple, quantitative tool for the determination of antibody half-life when immunochemical techniques are difficult to apply.

2. The half-lives of gamma globulin or antibody vary markedly from species to species and apparently even among age groups within a species.

3. The half-lives of gamma globulin or antibody seem to be dependent, at least in part, on the metabolic rate of the host.

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