An immunofluorescence study carried out on the relationship between the mode of localization of antigen and that of specific antibody and γ-globulin in the germinal centers of mouse popliteal lymph nodes during the secondary immune response. The germinal centers of popliteal lymph nodes from mice sensitized secondarily with antigen (human γ-globulin) contained both antigen and γ-globulin in a similar distribution, giving lacy or reticular framework of immunofluorescene. Specific fixation of heterologous (guinea pig) complement in the germinal centers was also observed in a distribution similar to that of antigen and γ-globulin. These results strongly suggest the presence of antigenantibody complex mechanism in the germinal centers. The functional significance of this mechanism in the immune reaction is discussed. Large parenchymal cells of the germinal centers were devoid of specific staining for specific antibody as well as for γ-globulin, which may support the view that the germinal centers are not a source of plasma cells but a site of lymphocyte formation