The claim that small, non-dividing lymphocytes carry the property of immunological memory is based on the observation that thoracic duct lymphocytes (TDL) from immunized rats will confer specific secondary responsiveness on heavily irradiated rats; and that destroying most of the large, dividing lymphocytes in the inocula does not reduce their immunological activity. However, it could be objected that a minor contamination with large lymphocytes may still have been responsible for the activity; and that, whatever the activity of small lymphocytes, large lymphocytes may also carry memory. Further, it is paradoxical that animals depleted of recirculating small lymphocytes can still mount normal secondary antibody responses. Lymphocytes from rats immunized to tetanus toxoid were fractionated by two methods and then assayed for their ability to transfer immunological memory. Velocity sedimentation separated lymphocytes on the basis of size. Normal TDL showed a continuous distribution of sedimentation velocities with a mode at 3.9 mm/h corresponding to a mean cell diameter of about 7 $\mu $m. The modal fractions consisted of > 99.5% small lymphocytes but fractions containing the faster sedimenting (larger) cells were always contaminated with small lymphocytes. Purified small lymphocytes and unfractionated TDL were equivalent in their ability to transfer memory. The faster sedimenting fractions, in which the large lymphocytes were enriched up to 300-fold, were always less active and such activity as they did possess correlated with the degree of contamination with small lymphocytes. Fractionation of spleen cells by velocity sedimentation showed that the persistence of responsiveness after lymphocyte-depletion was due to residual small lymphocytes. Fractionation of TDL by passage through glass bead columns gave highly purified small lymphocytes but very low cell recoveries. Small lymphocytes which had been purified by velocity sedimentation lost much of their ability to transfer memory after passage through such columns, probably because marrow-derived cells were selectively removed. The experiments overall show unequivocally that immunological memory is carried by small, non-dividing lymphocytes; they provide no grounds for supposing that large, dividing lymphocytes also possess this property.