White-pulp cells and whole spleen from donor mice immunized with sheep erythrocytes were transferred intravenously to heavily irradiated mice. The numbers of plaque-forming cells and the amount of hemagglutinating antibody produced after reexposure to antigen were measured. When reexposure to sheep erythrocytes was delayed, a much greater response occurred in the transferred cells. Peak responsiveness was reached at 24 hr after transfer. This "lag effect" was greatly reduced by repeated injections of 5-bromodeoxyuridine into the recipient mice prior to challenge with antigen. It was therefore concluded that much of the increase in responsiveness was due to a proliferation of "primed" cells after cell transfer. The fact that a significant response was given by the transferred cells in spite of 5-bromodeoxyuridine treatment suggested that some of the primed cells were nondividing. White pulp was a much richer source of responsive cells than was whole spleen.