Lymphocytes were obtained from the thoracic duct of rats 1½ to 15 months after primary immunization with a single dose of bacteriophage ϕX 174. An intravenous injection of these lymphocytes conferred on heavily X-irradiated rats the ability to form antibody in a secondary-type manner after a first injection of ϕX. Negligible responses were obtained after cell transfer if the recipients were not challenged with antigen.

Thoracic duct cells from some immunized donors were incubated in vitro for 24 hr before transfer in order to destroy selectively the large, dividing lymphocytes. The responsiveness conferred on X-irradiated recipients by such "incubated" inocula was then compared with that given by equal numbers of "fresh" thoracic duct cells. In all such comparisons the recipients of the "incubated" cells gave higher and more rapid antibody responses. It was concluded that the cells in thoracic duct lymph which carried immunological memory were small lymphocytes.

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