Polymerized flagellin from Salmonella adelaide was labeled with I(125) and injected into rats varying in age from 0 to 42 days. Lymphoid organs were removed at various intervals and the progressive development of antigen-capturing structures was studied using autoradiography. The chief findings were as follows: 1. Newborn rats lack the follicular and medullary antigen-trapping structures characteristic of adult animals. 2. At the age of 10 to 14 days, the first signs of specific cortical antigen localization appear in lymph nodes. This initially takes the form of a continuous "cortical rim" of antigen localization. 3. Within a further 4 to 6 days, the Anlagen of true follicular antigen-capturing structures appear, the continuous rim being only a transitional mechanism. 4. The antigen-capturing part of the follicle appears before the lymphoid component; follicle Anlagen can be defined only on autoradiographs and cannot be seen on ordinary histological sections. 5. The system of medullary macrophages develops gradually over the period 2 to 6 weeks of age. 6. The ability of lymph nodes to retain antigen increases progressively, there being a fivefold increase in the amount of antigen retained per unit weight of lymphoid tissue between 2 and 6 wk of age.