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Our major objectives were to determine (1) when mice develop the cellular mechanisms necessary to trap and retain immune complexes in lymphoid follicles, (2) whether the ability to trap and retain immune complexes in lymphoid and collagenous tissues was maintained in old animals, and (3) whether the pattern of antigen localization in lymphoid follicles was altered as a consequence of ageing. Mice were passively immunized with a standardized amount of specific antibody and then challenged in the foot pads with radioactive antigen. The results indicated that newborn, and 1- and 2-week-old BALB/c mice lacked the cells or cellular mechanism necessary for trapping, localizing, and retaining immune complexes in lymphoid follicles. By 3 weeks, however, this ability to trap, retain, and localize antigen became apparent. The ability to trap and retain immune complexes on tendons and in lymphoid tissues was maintained as long as 27-30 months. A comparison of the quantity of antigen retained per mg of tissue in young-adult (6 months of age) and in old (27-30 months of age) mice indicated that old mice retained slightly more antigen in lymphoid tissues and substantially more on tendons. Antigen retained in spleens of both young-adult and old mice was localized in follicles by 24 h. However, the antigen retained in lymph nodes of old mice remained in the subcapsular sinus and adjacent superficial cortex and was not localized in follicles even after a full week. The cellular mechanisms necessary to trap and retain immune complexes in lymphoid tissue appear to develop in BALB/c mice a few weeks after birth and persist throughout life. However, localization of antigen in the lymphoid follicles of lymph nodes diminishes in old mice. The antigen trapping capability of collagenous tissues was not only maintained in old mice but was substantially increased.