In this study, we investigated the expression and function of CD27 and CD70 in the human tonsil. We found that IgG production was significantly reduced after the administration of anti-CD27 and anti-CD70 antibodies in the T cell dependent B cell activation system, suggesting the involvement of CD27/CD70 in the production of IgG. In addition, the amounts of peripheral and tonsililar CD4+ CD27+ cells were almost identical, whereas the expression of CD4+ CD27- cells was higher (4.9%) in tonsil than in peripheral blood (2.4%). Furthermore, anti-CD27 antibody and anti-CD4 antibody-positive cells were found mainly in the interfollicular region, and a small number of this cell population was observed in both germinal center (GC) and mantle. Double-staining showed that the germinal center was almost completely composed of CD4+ CD27+ cells. In contrast to CD27 cells, only a small number of CD70+ cells were present in the follicles. Moreover, anti-CD19 antibody, a B cell marker, was stained in the GC and mantle zone, and CD19+ CD27+ cells were observed in the GC close to the mantle. The above findings favor the involvement and role of CD27/CD70 in the T-B cell interaction by inducing IgG production and increasing memory cell (CD4+ CD27-) number in the tonsil.