The immunohistochemical distribution of 15 blood coagulation factors in the germinal centers (GCs) of human Peyer's patches (PPs) was studied. Although factor VIII, active alpha-thrombin, and fibrinogen were hardly evident in the GCs, the majority of coagulation factors, such as kallikrein, high-molecular-weight kininogen, factors XII, X, IX, VII, V, XIIIa and XIIIb, prothrombin, anti-thrombin III and inactive alpha-thrombin were found, showing a lace-like staining pattern similar to that obtained with a monoclonal antibody, R4/23, specific for follicular dendritic cells (FDCs) in the GCs. By immunoelectron microscopy, positive reactions for factor X and XIIIa were found on the surfaces of FDCs, GC cells, and/or in the intercellular spaces of GCs, being especially marked on the surface of the labyrinth-like structure of FDCs. It is concluded that a majority of coagulation factors are localized in the GCs of human PPs. Furthermore, it is suggested that some of these coagulation factors have a close topographical relationship with FDCs.