The germinal center (GC) is the dynamic microenvironment where Ag-activated B cells rapidly expand and differentiate, generating plasma cells (PC) that produce high affinity antibodies. B cells within the GC have great heterogeneity, containing B cells at different stages of activation and differentiation. However, there are few surface markers that allow subsets of GC-B cells to be distinguished. In the present study, we show that GC-B cells in human tonsils contain two distinct populations regarding CD9 expression; CD9 and CD9+ cells. CD9+ GC-B cells are functionally more differentiated towards PC based upon the following evidence; (1) CD9+ cells express higher levels of PC transcription factor, Blimp-1 while lower levels of B cell transcription factors, Bcl-6 and Pax-5, compared to CD9 cells, (2) CD9+ cells differentiate into plasmablasts faster than CD9 cells in the presence of cytokines that generate PC, and (3) CD9 expression was induced in CD9 GC-B cells under PC generating condition and gradually increased in the course of PC differentiation. Taken together, our data suggest that CD9 is a novel marker for a human GC-B cell subset that is committed to PC lineage. (C) 2013 Elsevier Inc. All rights reserved.