A substantial portion of the lymphocyte-like cells in induced peritoneal exudates derive from cells which enter the blood by way of the thoracic duct. The migrant cells have been identified as large and medium lymphocytes, but they may also include short-lived small lymphocytes derived from them. Small lymphocytes which have a potentially long circulating life-span are excluded from exudates, although cells of this type predominate in thoracic duct lymph. The results imply that many (perhaps all) of the small round cells in inflamed tissue are members of a line of rapidly proliferating lymphocytes. Specifically committed lymphocytes with precisely these properties are added to the blood of rats infected with Listeria monocytogenes. The localization of committed lymphocytes in inflammatory foci could be the crucial event which enables the host to focus his cellular defenses at sites of bacterial implantation.