The purpose of this investigation was to address the current controversy regarding the T-cell requirement for the generation of B-memory cells. We have circumvented the possible objection to previous experiments regarding residual T cells in T-deprived animals by examining memory cell generation in relation to the numbers of T cells participating in the immune response. Thymectomized and lethally-irradiated rats were reconstituted with foetal liver or a more mature stem cell source, neonatal liver. These animals were given graded doses of purified T cells one day before immunization with alum-precipitated DNP-BGG + Bordetella pertussis. Four weeks after priming, cell suspensions from experimental groups were adoptively transferred to carrier primed recipients and challenged with DNP-BGG in saline to assess memory cell development. The data demonstrate that in the absence of T cells only minimal memory development occurred. However, when T cells were present, the level of memory cell development increased with increasing numbers of T cells. By examining the relative affinity of the antibody produced in the primary and secondary responses, the increase in memory cell development in relation to increased numbers of T cells was shown to be due to the selective generation of high affinity memory B cells.