AbstractAs part of our studies into the role of germinal centers, we investigated whether each de novo generated germinal center (GC) develops from one single GC precursor cell (GCPC, monoclonal development), a few GCPC (oligoclonal development) or from many GCPC (polyclonal development). Thus, lethally (9 Gy) X‐irradiated AO (RT1u) rats were reconstituted with 108 thoracic duct lymphocytes (TDL) containing mixtures of AO and AO × BN cells in various ratios. The AO TDL were tolerant for AO × BN cells by using TDL from AO →(AO × BN)F1 (RT1u/n) X‐irradiation bone marrow chimeras. To induce GC formation in the spleen of TDL‐reconstituted rats, animals were i.v. injected with 109 sheep red blood cells. Five days after reconstitution and antigenic challenge spleens were taken for analysis of cellular make up of de novo generated GC. Spleen sections were immunohistochemically stained with monoclonal antibody F17‐23‐2, recognizing major histocompatibility complex class II antigens of the RT1n haplotype but not the RT1u haplotype, to discriminate between B cells of AO and AO × BN origin. Analysis of the GC in spleens of rats reconstituted with a mixture of AO and AO × BN TDL revealed three types of GC: GC entirely composed of AO cells, GC entirely composed of AO × BN cells and GC containing a mixture of both. The relative frequencies of these three types of GC indicated that in our experimental system, de novo GC developed oligoclonally from one to three GCPC. These data strongly suggest that GC are sites of antigen‐driven expansion of peripheral B cells to very large clones.