The adoptive secondary antibody response of rats to the hapten-protein conjugate dinitrophenyl-diphtheria toxoid (DNP-DT) was used to investigate the migratory properties and rate of formation of T and B memory cells in the spleen. The experimental findings show that hapten (DNP-BSA)- and carrier (DT)-primed spleen cells act synergistically in the restoration of the adoptive anti-DNP response. Passage of both hapten- and carrier-primed spleen cells through an intermediate host (intravenous injection and subsequent collection in the thoracic duct lymph) showed that both cell types are able to recirculate from the blood to the lymph. In addition, memory to the hapten or carrier could be withdrawn from the spleen by prolonged thoracic duct drainage. The rate of formation of hapten- and carrier-primed spleen cells was studied by treating donors with [(3)H]thymidine for 48 h before cell transfer in an attempt to "suicide" rapidly dividing cells. Only a slight reduction in the adoptive response to the hapten or carrier was noted upon transfer of treated cells to irradiated hosts. In further experiments, the cell lineage of hapten- and carrier-primed cells was determined by treating each cell type in vitro with rabbit antirat B cell serum (RARBS) and complement. Although treatment with RARBS did not affect the adoptive response restored by carrier-primed cells, the same treatment abolished the response restored by hapten-primed cells. These findings indicate that T and B memory cells in the spleen of the rat are relatively long-lived, recirculating lymphocytes. The contribution of fixed or rapidly turning over cells to immunological memory is small or negligible as compared with the latter cells.