A technique for in vivo quantitation of transplantation immunity has been presented. Known numbers of H3-thymidine-labeled lymphoid cells were injected intravenously into isologous or homologous recipients. The total radioactivity in host spleens was counted at different time intervals. The rate of isotope disappearance from the spleen was exponential to 0.5 per cent of the dose or less. In isologous recipients, the calculated half-life of labeled cells was 82 hours; in homologous recipients it was 40 hours; and in presensitized homologous hosts it was 18 hours. The technique is a highly reproducible, precise, and sensitive measure of transplantation immunity.

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