The immunity elicited by 2 million intraperitoneally inoculated homologous spleen cells in mice is dissipated by the end of 60 days, as is evidenced by the epithelial survival scores of orthotopically transplanted skin homografts. If the skin test-grafts at this time are accompanied by a normally subliminal immunizing dosage of intraperitoneally injected spleen cells, however, their scores are significantly lowered. This is interpreted as showing that after primary immunity has faded the host's homograft rejection mechanism is in a hypersensitive state such that an immediate encounter with even an ordinarily negligible stimulus results in the reinstatement of an effective immune condition, i.e. , that it represents a true secondary or anamnestic response.
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