Germinal center (GC) B cells undergo proliferation, somatic hypermutation and isotype switching in the course of differentiation into plasma cells to produce high-affinity antibodies. To understand the molecular mechanism regulating the expansion of memory B cells and the termination of expansion by differentiation into plasma cells, we investigated the effect of interleukin-2 (IL-2), IL-4, IL-10 and CD40 ligand (CD40L) on the differentiation of GC B cells in the defined culture system containing a follicular dendritic cell (FDC)-like cell line. IL-2, IL-4 and CD40L are required for the optimum proliferation and differentiation of GC B cells. When IL-10 was added to this culture condition, CD20+ CD38+ GC B cells sequentially differentiated into CD20+ CD38- memory B cells and then CD20- CD38+ plasma cells. In the absence of IL-10, the resulting CD20+ CD38- memory B cells continued to proliferate and retained its phenotype. The proliferation of memory B cells was interrupted by addition of IL-10 which induced the differentiation into plasma cells. The expression of CD80 and CD86 was up-regulated in the memory B cells, compared to naive B cells and plasma cells. The identity of memory B cells generated in vitro from GC B cells was further substantiated since memory B cells generated in vivo displayed the identical pattern of proliferation and differentiation under the same culture condition. These results highlight the potent role of GCT helper cells in the expansion and differentiation of memory B cells by regulating different cytokine production.