The ability to culture and expand B cells in vitro has become a useful tool for studying human immunity. A limitation of current methods for human B cell culture is the capacity to support mature B cell proliferation. We developed a culture method to support the efficient activation and proliferation of naive and memory human B cells. This culture supports extensive B cell proliferation, with 10(3)-fold increases following 8 d in culture and 10(6)-fold increases when cultures are split and cultured for 8 more days. In culture, a significant fraction of naive B cells undergo isotype switching and differentiate into plasmacytes. Culture-derived (CD) B cells are readily cryopreserved and, when recovered, retain their ability to proliferate and differentiate. Significantly, proliferating CD B cells express high levels of MHC class II, CD80, and CD86. CD B cells act as APCs and present alloantigens and microbial Ags to T cells. We are able to activate and expand Ag-specific memory B cells; these cultured cells are highly effective in presenting Ag to T cells. We characterized the TCR repertoire of rare Ag-specific CD4(+) T cells that proliferated in response to tetanus toxoid (TT) presented by autologous CD B cells. TCR V usage by TT-activated CD4(+) T cells differs from resting and unspecifically activated CD4(+) T cells. Moreover, we found that TT-specific TCR V usage by CD4(+) T cells was substantially different between donors. This culture method provides a platform for studying the BCR and TCR repertoires within a single individual.