We developed a quantitative procedure to examine the ability of memory B cells to give rise to AFC, and to more memory B cells (propagation of memory) after antigenic stimulation. Sublethally irradiated (BALB/c × C57BL/Ka)F1 mice were given an excess of BSA-primed T cells, graded numbers of DNP-primed B cells, and challenged with DNP-BSA in saline. The anti-DNP response of these adoptive (intermediate) hosts measures the production of AFC. After a rest period of 8 to 12 weeks, spleen cells from these intermediate hosts were transferred to sublethally irradiated final hosts. The latter animals were also given an excess of primed T cells and challenged with DNP-BSA. The anti-DNP response of the final hosts measures the ability of memory B cells to propagate memory. The relationship between the expression of surface immunoglobulin isotype (IgM, IgD, IgG), and the ability to produce AFC or propagate memory was studied by purifying populations of IgM+, IgD+, and IgG+ memory B cells on the FACS. The experimental results show that IgM+, IgD+, and IgG+ cells give rise to AFC, but that only the IgD+ subset can propagate memory. This suggests that memory B cells lacking surface IgD undergo terminal differentiation after antigenic stimulation, but that those bearing IgD are capable of self-renewal.