The in vitro generation of memory cells reactive to transplantation antigens is described. Blast cells, obtained from rat lymphocytes sensitized on xenogeneic or allogeneic fibroblast monolayers, reverted to secondary small lymphocytes after transfer from the foreign sensitizing to syngeneic monolayers. These secondary small lymphocytes had a limited in vitro life span of 4–6 wk. They manifested properties of memory cells: upon re-exposure to fibroblasts of the sensitizing phenotype, the secondary lymphocytes adhered to the fibroblast monolayer and transformed into blast cells with cytotoxic activity. The response of secondary lymphocytes was rapid, compared to that of normal lymphocytes, and directed specifically against the primary sensitizing antigens.

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