It was demonstrated by autoradiography using 125I-labeled antigens that in the secondary response to bovine serum albumin (BSA), a considerable amount of antigen is localizing in the germinal centers (GC) of chicken spleen prior to the onset of antibody production. A similar early antigen capture, although of a lesser degree, was also noted when birds originally immunized with BSA were injected with human serum albumin (HSA) on a second occasion. Early antigen localization in GCs was scarcely demonstrable in the primary response. Since antigen capture requires specific antibodies, it was concluded that the antigen binding cells correspond with B lymphocytes of GCs formed during the primary immune response or with their descendants. On antigenic stimulus, these cells transform to antibody producing cells after one or more mitoses. GCs can therefore be regarded as memory units composed of cells committed to one and the same antigen. Cells of the thymus-system probably play a cooperative role in the antigen-induced formation of GCs.