Abstract
Thoracic duct cells from rats which have survived an infection with Listeria monocytogenes can confer a high level of antimicrobial resistance upon normal recipients. The cells which confer protection appear in the thoracic duct during the 1st wk of the immunizing infection, at a time when newly formed lymphocytes are being added to the lymph in substantially increased numbers. The protective cells differ in at least two respects from the majority of small lymphocytes in central lymph: they have a rapid turnover rate and a short life-span in the circulation. Evidence was also obtained that lymphopoiesis affecting the long-lived small lymphocyte, which belongs to the recirculating pool, is not increased during an acute Listeria infection.