CD27, which belongs to the tumor necrosis factor receptor family, is expressed on germinal center (GC) but not on naive B cells, suggesting an important function of this molecule in the regulation of the GC reaction. We described here the expression of CD70, which is the ligand for CD27. We observed that in most tonsils, CD70 is only expressed on part of the IgD-, CD38- B cell population, which have been described as memory B cells. However, in 10% of the tonsils tested, CD70+ IgD+ GC were found. The CD70+ GC B cells were small cells that also expressed CD44 and CD39, but were CD10- and CD38-, suggesting that they represent very recent immigrants that are in the process of forming a GC. The concordant expression of CD27 and its ligand CD70 on this primordial subset of GC B cells suggests an important role for CD27/CD70 interaction at this stage of GC formation.