Using a popliteal lymph node weight assay the graft-versus-host activity of lymphocytes from donors immunized with allogeneic tissue has been assayed by comparison with that of lymphocytes from nonimmune donors. When the donors were immunized against weak histocompatibility antigens (non-AgB) the specific GVH activity of its lymphocytes was increased. This increase was greater if spleen cells rather than thoracic duct lymphocytes were the source of the donor cells used for assay. The increase in GVH activity was also greater if the standard immunization procedure of two successive skin allografts was followed by three boosting injections of allogeneic lymphoid cells.
When donors were immunized against strong histocompatibility antigens the specific GVH activity of the donors' lymphocytes was slightly increased, was unchanged, or was actually decreased depending on the experimental situation. In donors rendered incapable of a humoral alloantibody response by whole body X-irradiation, immunization across a strong barrier was followed by little or no increase in the specific GVH activity of TDL. In the rat, as in other species, the increase in GVH activity after immunization is inversely proportional to the strength of the antigenic barrier involved.