Acquired immunity depends on proliferation and differentiation of antigen (Ag)-specific B cells in germinal centers (GCs) of lymphoid follicles in response to T cell-dependent Ags. Here, we studied the function of GC-associated nuclear protein that is selectively up-regulated in GC-B cells. B cell-specificganp-deficient mice were compromised in affinity maturation of hapten-specific antibodies against T cell-dependent Ags with retarded development of GCs. B cell numbers and development, serum Ig levels, mitogen-induced B cell proliferationin vitro, and responses to T cell-independent Ag were nearly normal; however, the mutant B cells showed a decrease in anti-CD40-induced proliferation and an increased susceptibility to B cell apoptosisin vitroandin vivo. B cell-specificganp-deficient mice showed a decreased frequency of variable-region somatic mutations, especially of the high-affinity type (W33→ L) in the VH186.2 region against nitrophenyl-chicken gamma globulin, whereas the class switching was normal. We conclude that GC-associated nuclear protein is necessary for generation or maintenance of B cells with high-affinity B cell Ag receptors during the maturation in GCs.